To get the most out of the compressor you'll need to learn side-chaining and how to use dummy clips as a side chain source is handy as well. Theres a million videos on youtube which could explain a million times better than I ever could.
Automation is when you draw in your device or track adjustments in session view e.g volume, limit levels, EQ adjustments etc..
If you want a particular part of the track to appear to hit harder then it actually does e.g. your drop, you can Automate areas like the compressor or volume, so just before it kicks back in, raise up the volume and then you can gradually automate a volume drop down by a few db's till the next hard hitting area.
Heres a decent article on how to use it.