The quick shot of Vader's exposed head in his meditation chamber is our first glimpse of this inhuman villain's humanity. It also hints at the revelation that Vader is Luke's father, which culminates in his eventual redemption in Return of the Jedi.
The scene happens in a flash: An Imperial officer approaches Vader as he's sitting in the chamber, facing the other direction. We see his human head, pale white and scarred. Then, just like that, it's over: Vader's iconic helmet descends and clicks into place over Anakin's head.
This strange image of Vader as half-man/half-machine is one that simultaneously freaks us out and makes us more sympathetic towards him. Remember: At the time of Empire's release, people didn't fully grasp that Vader was a person, much less Luke's dad. This image of Vader's exposed head, however, gives us our first hint that a man still exists beneath that mechanical skin.
This conflict—between Vader the evil machine and Anakin the good man—is one that becomes key to the Star Wars series. We'll eventually see this thread reach its conclusion in Return of the Jedi, with Anakin finally shedding Vader's skin and redeeming himself.