In general English, bio means a biography or biographical sketch.
In media terminology, a ‘Bio’, short for biography, is the small portion of your online profile that explains to new or potential followers who you are. All social platforms have some version of a Bio as they are valuable in attracting new followers with similar interests. When it comes to your Twitter strategy, your Bio is the first thing users see when they discover your profile and a good Bio can greatly improve how often you show up in keyword searches.
Also, in slang, BIO means “Biological (toilet) break”.