VESA is a mounting standard for TVs and computer screens that can come as a VESA monitor stand or arm mount. There are different VESA specifications, but most modern screens are either 100 x 100 mm or 75 x 75 mm (both sometimes called FDMI MIS-D). This refers to the distance between the four screw holes.
Fortunately, almost all modern screens that are found at reputable retailers are VESA compliant and will say so either on the box, in the manual/instructions, or even on the back of the monitor itself. You can also easily search your model online to find out if it is.
When a modern screen or arm says its fully VESA compliant or a universal monitor stand, it means it can fit both 75x75 and 100x100 mm variants. If your monitor only has one of these sizes, the arm will fit that one with four holes spare.