A BLtouch is an auto bed levelling probe. When attached to the print head, it can be moved in a grid pattern around the bed, measuring the height at each probing point and then creating a contour map of the bed. When a new print begins, this contour map can be applied to the first layer, moving the nozzle up and down as necessary to compensate for a warped or uneven bed.
A BLtouch is very convenient in that it removes the need to manually level your 3D printer bed. In situations where the bed is warped, manual levelling will never give a perfect first layer and an ABL probe is the only means to achieve this. Compared to other contactless ABL probes, a BLtouch physically touches the bed with its pin when probing. Therefore it doesn't care what the bed surface is or it's current temperature. A BLtouch allows the print surface to be swapped or changed without recalibration.