Total Bases only counts the bases a player earns through hits: a single is 1 base, a double is 2, a triple is 3, and a home run is 4. A walk, error, balk, fielder's choice, steal, or passed ball doesn't count as a base.
For example: a player walks, steals second base, and then scores a run. Even though they scored, they recorded 0 total bases, so an Over 0.5 Total Bases bet loses.
For more information, see the General Rules, Tickets & Wagers article.