Smart HUD - What is the number next to my name?

This is part of the Smart HUD stats, the stat for VPIP, which stands for Voluntary Put-In Pot.

This indicates how often the player voluntarily puts money into the pot. The higher the number, the more often the player contributes to pots and thus plays hands. 
It is calculated as follows (Number of times the player voluntarily puts money in the pot preflop/total number of hands in sample) x 100