A bitch in heat literally means a female dog who is in heat. (Heat meaning she is 'in season' to mate and have babies.)
A bitch in heat is sometimes used when reffering to humans, generally females. A female ththat goes after every male she can find, and is being extremely lustful. A blunt way to put this would be a horny woman.
Literally: "The bitch was in heat."
"She was eyeing the man like a bitch in heat."
(Or sometimes it is written as "like a bitch DOG in heat.")