Total Bases measures the number of bases a batter gains on their hits which is calculated as:
TB = 1B + 2*(2B) + 3*(3B) + 4*(HR)
Total Bases only takes into consideration the bases gained through hits and doesn't include bases gained as a baserunner, stolen or due to another batter.
More about Total Bases Leaders
A player is given one total base for a single, two total bases for a double, three total bases for a triple, and four total bases for a home run (total bases are part of the computation for a player’s slugging percentage). Total base leaders is strongly correlated with extra base hits leaders, though if there is a player with an unusually high number of singles in a season he could rank among league leaders as well. Total bases does not take into account times reached by base via a walk or hits batsman. In recent years reaching approximately 350 total bases will often put a player among the league leaders. Slugging Percentage uses Total Bases in its calculation as SLG is formulated as total bases divided by at bats.