How does health insurance work?

I just started a new job, which offers health insurance. The specifics of the plan are incomprehensible to me. What I do know is I went to a health clinic for STD testing, something I’ve done routinely for the last five years, once a year or when dating someone new, through my student health plan without significant cost. My plan apparently doesn’t cover this sort of thing, so I owe 76 for the visit. This is stunning. I need to understand my plan better so that I can make an educate decision if I should happen to find myself legitimately ill in the near future. The company’s website offers nothing in the way of specific explanation, just the usual gibberish.