A "session beer" is something that a bunch of guys drink after work, with most or all of them buying a round. It's got to be relatively low in alcohol or people are going to get real sloppy. Maybe calling out the next day with the bottle-flu.
I like them and try to make at least one every year. I can drink session beer all day and not get drunk. If I want to get drunk there's liquor, wine, or stronger beers. For drinking all day without losing your head there are session beers.