This can refer to many things, like the second letter of the greek alphabet, software in development, or a person.
In the
case of a person, a beta male is
submissive, clingy, indecisive, weakminded, and unconfident. He can't deal with confrontation or challenges. He lets things happen instead of taking action.
When it comes to women, he either sucks up to them and loses their
respect, or is too afraid to pursue them. If he does
get a girlfriend, he makes too many demands on her, and either drives her away or becomes locked into servitude. The hoverhand is one of the surest signs of a beta.
Overall, he is the opposite of an alpha male.