What does it mean if you have itchyness and redness at where your underwear line goes?

You could be allergic to laundry soap, the underwear could be too tight, you could be allergic to the fabric, you might need to keep that area cleaner, you might be allergic to your bath soap.

Do any of those make sense?

Yeah they do thanks x

You are probably allergic to the elastic on those underwear. Try 100% cotton underwears for a while, and see if this makes a difference.