FDM is the abbreviation for Fused Deposition Modeling. In FFF, material (in this case: thermoplastics) is deposited in layers to create a 3D printed object. During printing, the plastic filament is fed through a hot extruder where the plastic gets soft enough that it can be precisely placed by the print head. The melted filament is then deposited layer by layer in the print area to build the workpiece.
There is a broad choice of FFF 3D printers for every budget, starting at a few hundred dollars. Filament rolls are relatively inexpensive (from $25 per kilo). These factors made FFF printers so popular among makers and home users.