A robot entering an unknown space faces a chicken-and-egg problem: to know where it is, it needs a map, but to build a map, it needs to know where it is. Simultaneous localisation and mapping solves both at once, updating its position and its map as it moves and senses.
It blends readings from cameras, range sensors and wheel movement, constantly correcting for error. The result lets a robot navigate somewhere it has never been, which is the foundation for anything that must move purposefully through an unfamiliar environment.