A Busker is someone who makes a living entertaining audiences in some way (music, comedy, amazing tricks,etc) and instead of charging a fee they pass their hat.
At the end of the show the busker will usually pass the hat, (unless they are a music busker in which case they may have a hat out in front of them whilst they are playing). Buskers leave it up to the discretion of their audience to judge how much they are worth.
Busking is one of the oldest and fairest professions in the world, it doesn't matter if you have no money or are a millionaire you can come and watch a busking show for free, however this is a job based on trust, and the buskers trust that if they entertain you, you will support them. Therefore they can keep entertaining your children and your children's children.
So be as generous as you can, the only people that pay the buskers are the people that have enjoyed their shows!!!