The fuser is the part that melts the toner onto the paper. After the toner is transferred onto the paper, the fuser applies heat and pressure to ensure that the toner stays on the paper permanently.
The fuser rollers heat the page after the toner is applied, so the toner partially melts and sticks to the page for a permanent bond.
The part of the printing system that forces the toner to adhere to the paper. There are several types of fusing systems (heat, chemical, and pressure).