Just to be sure we're on the same page. Access 2002 is pretty old and has not been considered a viable version since Access 2003 was introduced, and that too is long out of date. If that's the only option open to you, so be it. But I would strongly consider upgrading.
So, the validation rule is one of the possible properties you can define for a field in a table. It is an expression which controls what kind of values can be added. E.g. perhaps it would limit values in a number to only positive values.
It's possible you've accidentally typed something into that property, and that is what causes this problem.
