A business angel is a private investor, who uses their own money (as well their network and, often, their own time) in startups on the pre-seed and seed investment stage. These stages are usually the riskiest - only 40% of startups manage to get to next investment rounds. Typically, angels sell shares back to founders or venture funds when startups are about to go to the A round.
As very early-stage investments are risky, angels usually ask for a rather large share of the company they invest in - from 25%.
Typically, a business angel is an experienced entrepreneur himself, preferably with experience and connections in the field in which he invests. In countries with a developed startup and angel investment market, there are associations that help inexperienced people who have free money, but do not have entrepreneurial and investor experience, become “passive” investors - invest in startups without going into their product and developing them.