What does the title mean? Who is Offside?

When I read that the penalty involves an offensive player who does not have the ball moving between the defenders and the goal (sneaking) - it is apparent that the women are "offside." When they sneak in to the stadium they try to achieve their goal of watching professional sports by getting past the government defenders - the police and the army. But the women can't play by the rules here because they aren't allowed to participate so all they can do is cheat and hope they don't get penalized.

I don't know much about football. How common and how trivial or serious is an Offside penalty?