Tickling a Pool Table

This is a classic pun. The joke plays on the double meaning of "balls" which refers both to the balls used in the game of pool and to testicles. Tickling is associated with laughter, therefore, the implication is that the pool table is tickled in the 'ball' region causing it to laugh. It's a bit cheeky, playing on a slightly suggestive interpretation to make you chuckle.