Polytetrafluoroethylene (PTFE) is a synthetic material, similar to Teflon (and closely related to it). Thanks to their low friction coefficient, tubes from this material are used to guide filament in all Original Prusa i3 printers. Not all the tubes are the same, multi-material printers often require narrow and more precise tubes, than the single-material printer. Before your order or make a new tube, make sure you have the correct dimensions.
In general, we distinguish between :
A) the tubes for the hotend, which are inside the extruder (common for all printers)
B) the tubes guiding the filament to the extruder (mostly for the multi-material printers)
Most of the tubes can be easily removed using your hand. However, the tubes in hotends require a partial printer disassembly. Learn how to change/inspect the PTFE tube in hotend for the MK3/MK2.5, and for the MK3S/MK2.5S/MMU2S.
In case your printer is jammed and the tube is damaged, you have three options: