Many DSLRs are derived from 35mm film camera systems that use 24mm x 36mm frames. Currently, the majority of DSLRs (including the D70s, as far as I know) have image sensors that are smaller. "Full frame" would mean the camera has an image sensor with an area of 24mm by 36 mm.
This is of course a completely arbitrary reference. There's no deeper meaning to it, and a camera doesn't magically yield better pictures when its sensor size increases from 23.9 by 35.9 to 24 by 36 mm^2.
