Bi erasure involves a lot of small things that basically ignores the existence of bi people. It goes from saying that bi people don't exist to more subtle things like assuming that everyone in a relationship with a man only likes men and anyone in a relationship with a woman only likes women. Or seeing a female friend who used to have a girlfriend and now has a boyfriend and saying "oh, you're straight now?" rather than considering they may like both.

Usually bi men are assumed to actually be gay, and bi women to actually be straight (I guess everyone must secretly like men only and only pretend to like women?).

It also can involve more systemic things, like characters in stories not being bi even if they have relationships with people of multiple genders, being instead portrayed as having a fluctuation orientation, changing their mind or finally coming out. Or there is also this idea that a bi person will have a "real" relationship with someone of the opposite gender but that anything with the same gender is less "real", not an actual relationship, can't possibly be serious and is just for fun.