Some say people use alcohol to self-medicate. What does this mean?

When I drink alcohol it is for two purposes: To relax and to forget my day. Does this mean I self-medicate when I drink because I have a ****** day and I want to relax and forget about it? When does it cross the line to unhealthy? What does it mean to self-medicate?

Also, I don't want to make two posts. Sometimes when I am emotional I'll drink enough to where I start screaming. It calms me down to scream, and I am always alone when I do this so that I can just scream about my fears and let it all out. Is this normal?