FBW is where the movement command for a control surface is an electronic signal rather than a mechanical linkage (control rod). This signal is usually generated by a computer which in turn signals an actuator which moves the servo valve on a hydraulic actuator which moves the control surface. There are several advantages to this, the main one being the weight saving of dispensing with long bits of metal running through the plane.
FBW a/c also tend to have advanced stability and control laws on them which shape the pilot's (or autopilot's) inputs to give a desirable control response. This means that the pilot's perception of how the a/c flies may be very different to how it would fly with conventional controls. Indeed, many modern a/c (especially fighters) are unstable without the control laws running.
Hope this helps,
VTOL