I am not sure how best to explain this but the clitoris is the female equivalent of the male penis. Basically, at foetus level, everyone starts out as female but as time goes on, those with XY chromosomes change developmental path to become male. If you are a boy, then then what would have been the clitoris if your were a girl instead grows into a penis. Basically, both organs are a group of highly stimulatable nerve endings and upon arousal both become engorged with blood and erect. As the clitoris is closer to the size of a pea (with some variation), when we say it erect, it just means that it is swollen and more pronounced than anything else. Obviously, because something that is more nodular rather than cylindrical can't actually be "erected" by true definition in the way that a penis can.
Does that kind of make sense???