Lets say that you are working on a feature branch on which you also fixed something critical in one of the commits. Lets assume the commit hash is 7e877d2. The fix in this commit is required in another branch, say master.
To do this,
1. Checkout master
2. Cherrypick the commit
git checkout master git cherry-pick 7e877d2
This just applies the changes in the mentioned commit to your master branch.