E-TPS stands for Encounter-TPS. It shows you the average TPS produced over the course of an entire fight. This differs from normal TPS, which instead calculates your TPS as produced by the last 15 threat producing actions you did.
Encounter TPS is the threat/time from PLAYER_REGEN_DISABLED to PLAYER_REGEN_ENABLED. It's less useful than the normal TPS column, which shows you your local TPS, and may help you determine incoming problems. E-TPS is more of a benchmarking statistic, to allow tanks to evaluate their average threat output for a fight, and see how that changes with gear, talent, or strategy modifications.