When landing what do the numbers said by the plane's computer mean? They are sometimes followed by retard, retard. Is this the altitude or is this the feet away from the landing point on the runway? Is there a way to know exactly where the plane should land on the runway? I am a complete newbie but love the airplane vids on youtube. Thanks.
Those are height callout, typically based on radar altitude, therefore measuring your height above ground. E.g. for the Airbus A320:
Automatic Call Out
FWC generates synthetic voice for radio height announcement below 2500 feet. These announcements come through the cockpit loudspeakers even if the speakers are turned off.
Predetermined call out
The altitude call out uses the following predetermined threshold:
The loudspeaker announces RETARD at 20 feet or at 10 feet if autothrust is active and one autopilot is in LAND mode.
(Airbus A320 FCOM - Navigation - Radio Altimeter)
For more information on RETARD, see Why does the A350-900 call out to retard at 20 feet above the runway?
The call outs are not always based on radio altitude. E.g. on the Boeing 737 NG the 1000 and 500 call outs are based on barometric altitude above landing field elevation (see this answer for details).
Why, of course it's assessing the pilot's IQ and medical condition!...
...Either that, or it's calling out the altitude in feet, followed by an instruction to retard thrust levers to idle, which is required for either wheel brakes only stop or thrust reverser deployment.
Most pilots choose to go with the second version, which also allows them to fly again.
The numbers are the radio altimeter call out - it's a very precise way of measuring height above ground, and the callouts are there to aids the pilots with their situational awareness as they approach the ground.
'Retard' is an airbus specific callout which is instructing the pilots to reduce thrust, as they approach the runway