A semicolon is a colon and a comma mixed together; see what I did there?
You use it to join two clauses which could be two standalone sentences. However you want there to be a slight pause, a skip of the beat as it were, instead of a period between them. I’m not sure how else to explain it.
It’s not that hard to explain; you could have just done it like this.
I think you look beautiful tonight; your sister is hot af though.
I said that to my wife once; I’m single now.
I said that to your wife once, too; neither of us are single now.