BPM is an estimate of a player's points contribution relative to a league-average player. The metric was developed by Daniel Myers as a way of evaluating a player's value to a team in a single statistic that synthesizes the traditional box score statistics. A BPM value indicates how many points above or below league average a player contributes per 100 possessions, so the league average is defined as a BPM of 0.0. An in-depth breakdown of BPM can be found in
this post by Daniel Myers.
A player's BPM can be found on the right end of the Advanced Stats table. This can be found by navigating down a player's profile page to the fifth table, or you can directly access it by clicking "Advanced" under "On this page" in the sidebar menu.