Imagine the mood of most people represented as a sine wave graph made up of peaks and valleys, cycling up and down over time. Then imagine the graph of people diagnosed with clinical depression as having valleys on that graph that sink down way low, abnormally low, into areas beyond the average person's "the blues" or "my life kinda sucks and I'm bummed about it," and into "it feels like my brain is suffocating me and I can't experience normal emotions" and other such poetic analogies to describe the exotic weirdness the compromised brain can descend into. Anti-depressants are supposed to cut off those abnormally-low valleys, letting your mood sink only so low. They're not supposed to make you feel like everything's groovy or that you're perpetually high on life or sth. So if your life sucks (independent of your mental illness) you'll still feel bad, but you'll ideally feel only as bad as most normal people would.
Anti-depressants also supposed to help people who have off-the-charts gnarly mutant depression, with symptoms like not being able to move, loss of bowel control, and hallucinations.
I know a little about this--my avatar is a sitcom psychiatrist.