The Crew Escape Tower is a small stage rocket part placed on top of a Crew Capsule or Crew Capsule Dockable used to increase aerodynamics and as a means of launch abort.
Ascent to orbit without this is still possible without the crew module heating up.
The crew escape tower is also used when the first stage is compromised and the rocket starts falling. When this happens, it will blast the crew capsule away from the rocket, but the crew capsule is not visible under the tower.