can someone give me an explanation on what it means or how it works when someone is giving another opponent a set amount of bb/100? particularly in a heads up challenge.

It means that one player is giving the other player an amount of money to make up for the perceived difference in skill between the players.

Bb/100 is big blinds per 100 hands. So if Player A is giving Player B 2 BB/100 and they are playing $2/$5. The Player A is giving Player B $10 (2 × $5) for every 100 hands they play. So if they play 10,000 hands then Player A would pay Player B $1000.

Obviously Player A thinks he is good enough to make more than the $1000 he is paying Player B when they play 10,000 hands heads up. Player B probably knows Player A is better than him, but he thinks the $10 per 100 hands will make up for the difference in skill.

It is just a way of having two people of different skill levels still be able to play against each other.

thanks, this was a good answer, it sounds similar to giving someone strokes in golf to makeup for a difference in handicaps.