Your movement speed is increased by 15%. This does not stack with most other movement speed increasing effects.
Fleet Footed is a passive core rogue ability learned at level 62. It increases the rogue's movement speed by 15%, but does not stack with other movement speed increasing effects.
This can actually be considered a DPS increase. The less time spent running away from fire, poison, oozes, or other unhealthy stuff, the more time spent attacking.
Patch changes
Patch 5.0.4 (28 August 2012): Movement impairing effect resistance removed.
/Patch 3.0.2 (14-Oct-2008): Fleet Footed (Assassination) now reduces duration of movement impairing effects by 15/30% rather than increasing resist chance.
Patch 2.3.0 (13-Nov-2007): Now increases movement speed by 8/15%, up from 4/8%.