When you get a pawn to the eighth rank, that pawn can/must be promoted to a piece. As a rule, that will be to a queen, but in rare cases it may make sense to promote it to a different piece.

 

Make sure that in your browser the blocking of pop-ups has been deactivated. The reason: in a browser dialogs are often blocked as though they were pop-ups. Therefore under-promotion is implicit: you move the mouse over the target square and what appears there is a large queen and to the right of it the other pieces. If you let go of the mouse too soon and over on the right the mouse pointer might be over the wrong piece and the result is then an under-promotion you did not want!