Don't feel dumb - you know what they say about dumb questions...
Anyways, I'm not entirely sure of how the term is used in the cases you are thinking of, so this may not exactly cover it. However, I think in many cases it is used to suggest that a particular part or assembly was sourced from another kit - for example, 'the donor parts came from my XXXXX kit...'
Of course, that is just my opinion..... I could be wrong...