The ACA is a massive bill that affects basically every part of healthcare in the US. That being said, here are some of the major parts that affect people who get their coverage through their employer:

1. Minimum coverage standards - The ACA sets minimums about what must be covered by employer insurance, including drugs, procedures, family planning and mental health care.
2. No lifetime caps - Before the ACA insurers could set a lifetime cap on how much they would pay out for any individual. This meant that people who had long term chronic or very expensive medical needs would get kicked off their insurance eventually and have to figure something else out. Or, more likely, either go into massive medical debt or forego care, or both.
3. Pre-existing coverage - Before the ACA insurers could choose not cover issues that you got before you signed up with that insurer. So, again, if you had a chronic condition and changed jobs, you could lose all coverage for those treatments.

There is probably a lot more, but those are the big ones for most people.