Hey! Don't be sorry to ask, always cool to help out.
The maintenance. Well, one in a while you will need to replace the tubes. The tubes look a little bit like a long clear light bulb, and they wear out somewhat in the same manner as a light bulb.
There are two "types" of tubes, preamp and poweramp. Preamp tubes are smaller and require no warming up. They can last many years, but they gradually start fading away. If you gig / rehearse very often try replacing these every two years. They'll easily work for 15 years but the tone gets worse and worse and you won't really notice it because it happens so gradually. Try replacing them every two years anyway.
Then there's the power tubes. These are bigger and require to be warmed up before you put them in operation. If you don't warm them up you lose a lot of life on them, so they wear out faster. If you look inside a tube you will notice that on the bottom there's a wire much like the one inside a light bulb glowing red. This is the one that heats the tube up.
To do everything correctly before you press the power switch make sure the "Standby" switch is in the off position. Then flip the power switch. Now, wait as long as you can be bothered before you flip the standby switch. 15 minutes is very safe, 10 minutes probably too, 5 minutes is okay. I usually don't have patience to wait more than 2-3 minutes and my power tubes are fine, but i would recommend you wait longer.
When you turn it off it's good practice to first turn off standby then turn the power off, so that next turnon the standby will be in "off" position. You don't need to turn standby off, but do it anyway.
Bias. The signal that goes out from your guitar varies in voltage. Voltage is in electrics the same thing that pressure is in water pipes, and is measured in Volts. There's also a thing called current, which is the same as flow in a water pipe, which is measured in Amperes. If you multiply current by voltage you get power, which is measured in watts.
Now, the thing is, tubes block all signal going through them unless it's higher than a given voltage (for simplifying things, let's say it's 1 volt). So, since your guitar signal varies in voltage (let's say from 0 to 5 volts), the tube won't pass any signal until the voltage goes above 1 volt, rendering you unable to hear the signal below 1 volt. This results in what is called "crossover" distortion, which some people find harsh to the ear.
The solution to the above issue is to provide a constant signal to the tubes, so that the tube is always open, letting signal flow through it. It doesn't matter if the voltage from your guitar is 0.1 volts, it will still pass since there's a constant voltage opening up the tube to a signal of any strength. This constant signal is called Bias, and might be measured in current, voltage or watts. It doesn't matter, you just need to find out what to set it to.
How to find out:
To set the bias (in current) on the power tubes right you need to know how many watts one tube can put out and the voltage they run on. Usual value for the power tube is either 25 watts or 30 watts (EL34 and 6L6GC respectively). Operating voltage is commonly 450 volts, but it might be more or less (my 5150 runs at roughly 470 volts). Usually the bias power is set to 60-70% of the maximum power.
So. Max power is 30 watts per tube. You want 60% of that as the bias power, which is 18 watts. Divide 18 watts by the operating voltage, which is 450 volts. Result will be 0.04 amperes, or 40 milliamperes (mA). Set the bias current to 40mA using your bias meter, and you're good. (By the way, 30 watt tubes usually run on a bit higher voltage so that the amperage will be less, but this was just an example).
Preamp tubes bias themselves, you just rip them out and put them back in without problems.
100 watts of tube power is very, very loud. Using more speakers and more efficient speakers also makes the sound louder. A 100 watt tube amp with a 4x12 cab is pretty much earth-shattering.
Stereo cabs are cabs that are wired so you can run one amp on one side of it and another amp on the other side, or use a stereo amplifier with it.
A rack is a cabinet you can put things in, like effects, a noise gate, an amplifier, a preamp, a poweramp, a wireless system etc. Things that are made for a rack is standardized in size so they'll fit.
A preamp is an amplifier that amplifies the voltage from your guitar so that it's high enough to run the power tubes (a guitar by itself would be too weak). A preamp also generates distortion, and provides sound sculpting tools like an equalizer and a gain and volume control.
On the other hand, the power amp amplifies current, to make your speakers push air. A power amp can also generate distortion and have sound sculpting controls like a master volume and Depth/Resonance and a Presence control.
For the record most distortion in metal comes from the preamp, with very little or no power amp distortion. Some metal styles also use power amp distortion to a large degree, and in rock'n'roll there's generally very little preamp distortion and a lot of power amp distortion.
To get preamp distortion, turn the gain up. To get power amp distortion, turn the volume up