The bowden tube is the plastic tube usually white PTFE, that the filament runs in from the reel to the extruder and or the hot end, and the end where it gets hot deteriorates and causes problems with melted filament leaking above the nozzle, the same thing happens if the end is not cut square.

If one is not already fitted, an all metal hot end with a proper heat break is a lot more hassle free and not prone to heat creep.