The pass transistor drops the voltage down to the required level and allows high current to pass. Often a bipolar transistor in an emitter-follower connection.
It should have a reasonable gain to allow a small base current and be able to dissipate enough power (voltage drop multiplied by current drawn), and of course be able to withstand the required current.
P.s. I've just noticed that you have asked this question twice - I would suggest deleting the other copy. Also, this question would be better placed in the "Elementary Electronic Questions" forum.