For example, this question, has the following diagram
M1 - M2 - M3
A1 - A2 A1' - A2'
It's a symbol borrowed from the mathematical notation of a derivative.
A1', in this example, is the result of rebasing
A1 on top of
M3. While they will probably have the same body and same commit message (assuming there were no merge conflicts during the rebase), they have different parents (
A1's parent is
A1''s parent is
M3), and thus they are not the same commit, and do not have the same commit hash.
A1' is derived from the original
A1, and this common mathematical notation is used to highlight it.