It may just seem like a normal word but it has a deeper meaning. It's a phrase that you can use for basically any emotion. Saying forehead is kinda like saying "spill the tea" or "wig" with the difference that you can use it in almost every occasion, with your friends, your family and pretty much everyone. Also it is mainly used at the end of a phrase.
Omg, how dare she steal my wig! Ugh... Forehead.
Are you for real right now!? Forehead....
Um you can't come with us... Jesus.. forehead...