One of the two main ways of integrating changes from one branch into another, in git, is to do a merge. The flag --no-ff is used in the context of performing a git merge operation; it stands for 'no fast forward'.

To understand this flag properly, the notion of merging a branch - doing it in a fast-forward way, and otherwise - should become clear. This article: ariya.ofilabs.com/2013/09/fast-forward-git-merge.… ... explains the difference in a succinct way.

Bonus: The other main way is to do a rebase Check this out: git-scm.com/book/en/v2/Git-Branching-Rebasing

From --help:

--ff When the merge resolves as a fast-forward, only update the branch pointer, without creating a merge commit. This is the default behavior.

--no-ff Create a merge commit even when the merge resolves as a fast-forward. This is the default behaviour when merging an annotated (and possibly signed) tag.

If you merge fast-forward the existing "source"-commits are "placed" on top of the "target"-commits. With --no-ff a (single) new commit is created and "placed above" the last "target"-commit.

PS: A good git-resource to read is the GitBook. (in different languages available)