After developing a feature or fix you want to merge that branch onto the parent branch. There are two main different approches: One creates a linear and the other a non-linear history.
Scenario
c1b0---62d8 [feature]
/
e73a---6cfa---90ed [develop]
Linear history
Rebasing before the merging, without creating a merge commit.
Result
e73a---6cfa---90ed---c1b0*---62d8* [develop]
* Those commit hashes are actually different, because of the rebase.
Non-linear history
Merging without rebasing before, creating a merge commit.
Result
c1b0---62d8 [feature]
/ \
e73a---6cfa---90ed---2e1g* [develop]
* This is the merge commit, necessary because of the missing rebase.