Hi Joe C,
Thought i could help out too. Nice to see you're getting into synthesis, it's a long and complicated path, but one that is ultimately very rewarding.
Best way to think of an envelope generator is as a controller. It can control certain parameters of the synth, but the most common are Volume (to shape the curve of the instrument's loudness) and Filter Frequency (for the auto wah type sound).
In most basic subtractive synths you'll get at least one envelope generator usually called AMP ENVELOPE which is automatically connected to volume, and maybe a second usually called MOD ENVELOPE, which can be assigned to any parameter you choose.
ES E is probably not the best synth to start with. If you're definitely interested in learning synthesis, i can highly recommend the ES2 tutorial. You'll learn everything you need to know to use ANY subtractive synth in the future, including ES2, which is the my personal favourite synth in Logic.