What is a fuser unit?

On a color laser/LED printer the image is created by toner particles that are put on the paper by an electro-static process. To bond the image with the paper the toner particles must be heated under pressure. This is what happens in the fuser unit. Two hot rollers pressing against each other melt de toner onto the paper. Or better put: It fuses the toner onto the paper, hence the name: Fuser.