A character actor is an actor who plays a particular kind of character in a play or in a movie, opera or ballet. The character actor is not the main part. He is not the lover or hero. He has a character which helps to support the main characters and therefore the story.
For example: in The Wizard of Oz the main character is Dorothy. The Scarecrow, the Lion and the Tin Man are character roles.
Some actors or singers prefer character roles to main roles. It may be because this kind or role suits their personality or their singing voice. Some people nearly always play the same kind of character role. We say that people like that are "typecast". Some actors are happy to remain typecast. Others may wish to broaden their repertoire and take on different roles.