The scene you're asking about is from Season 3, Episode 4, "Isolation".
In this episode, Thomas (a member of the group from the prison) and Brenda (a member of the group from Woodbury) are trapped in the prison's basement. They are surrounded by a horde of walkers, and Thomas is trying to protect Brenda.
Thomas doesn't actually kill a walker, but he does try to stop one from attacking Brenda. He slams a pipe into the walker's head, and while it doesn't kill it, it does distract the walker long enough for Brenda to escape.
This moment is significant because it highlights the fear and desperation of the characters. It also shows the lengths to which they will go to protect each other, even in the face of seemingly insurmountable odds.
It's important to remember that while the show uses the term "crank" to refer to walkers, it's not actually an official term. It's a colloquialism used by some characters in the show.
If you have any other questions about this scene or any other part of the show, feel free to ask!