What did a lion on a shield in the middle ages mean?

It simply meant that the arms allocated to that particular knight, or chosen by that particular knight, included a lion. There was no special significance in choosing a lion, or a chevron, or a cross, or a fleur-de-lys, or any other design.

The lion itself had no meaning unless it was a visual reference to the name of the knight - the arms of the modern Bowes-Lyon family include bows and lions used in exactly this way.