If you recorded from the TV on to a DVD-RW and finalised it then, as VHG says, you cannot add anything else to it, but you can put the disc back in the same recorder and un-finalise it so that you can add further recordings.
At least you can with my recorder and I assume that they are all similar these days.
If you record from the TV and do not finalise it then it can be played on the original recorder but you could not play the disc on a computer should you wish to do so.