Semiformal means not very formal, but also not very casual. It's in the middle.
Semiformal is like when you dress up a little bit, but not too much. It's like when you talk nicely, but not too nicely.
Examples:
The party is semiformal, so wear nice clothes, but not too nice.
We have a semiformal meeting, so don't wear jeans.
She has a semiformal way of talking.
He wore a semiformal hat to the party.
The cat had a semiformal look, with a little bow tie.
The semiformal dance was fun, but not too fancy.