A "cell" is the smallest native storage unit in most Forths. In Reva in particular, it is a 32-bit wide value. Unlike "C" and most other high-level languages, in Reva a "cell" has no particular "type" associated with it -- it can represent a character, a number, a pointer to some other data... whatever the programmer decides it should be.
A "stack" is the fundamental data structure in Forth. It consists of "cells", which are (so to speak) stacked up one on the other. Forth "word"s operate on the "stack" to perform whatever needs performing; they pull parameters off the "stack", and leave their results there for the next word in line.