VESA is a set of standards for the manufacturing of flat screen televisions and PC monitors. Most manufacturers comply with this standard by adhering to an industry wide "screw hole pattern" on the back of their displays for mounting purposes.
A VESA hold pattern is measured in both horizontal and vertical distance between mounting holes. For instance, VESA 200 x 200 means your TV mounting are 200mm apart horizontally and 200mm apart vertically.
N.B.: There are the manufacturers today are not using standard VESA holes. Please double check your television's manual before you purchase any mount.