Even though I've been playing guitar for a year and a half I still don't what difference to the tone is if you fiddle with the high, low, mid, reverb etc.

Could someone please tell me what difference they make to the tone?
It's pretty self explanatory:

Low = increases low frequencies = bass sound
Mid = increases mid frequencies = rich, full sound
High = increases high frequencies = bright, crisp sound

Why don't you try to mess with the settings of your amp and hear the differences?
The Low/Bass, Mid and High/Treble knobs are the controllers for the EQ section of your amp. You can use it to sculpt your rig's sound to a tone that suits the style of music you're playing. If you sit down with your amp and play chords whilst turning those knobs way up and down, there should be a very noticeable difference in the tone of your guitar.

The reverb knob adds reverb (duh), an effect used to emulate the sound of your amp being in a large room or chamber. Turn it all the way down, play a little, then turn it all the way up and play more, and you'll notice the difference.
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it depends on the amp.

on an old plexi, they didn't do much.

other amps, they do too much and push the amp into ugly territory.

you have to turn them to see.
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