Keep in mind when modifying or adding new files on different branches, if there isn't a commit, those changes will “propagate” to other branches if you switch. When you’re working on a branch and you're not quite finished but need to move onto another branch, git stash will take all the changes in your working tree and index and “stashes.” You'll run git stash apply to “unstash” the changes. Keep in mind it is applied to the current branch.

When you perform a stash, it's a merge commit. Git keeps track of the state of the index/staging-area and the working tree. Keep in mind that the index and working tree could contain changes to the same file. So essentially there are two commits when you stash. With these two commits, Git is able to “unstash” your changes.

# This command will run "apply" and a "drop" to keep your stash list clean.
git stash pop

git init
Initialized empty Git repository in /home/john/Projects/git-stash-test/.git/
 
echo "bacon" > bacon.txt
git add bacon.txt
git commit -m "initial commit"
[master (root-commit) 6d2649a] initial commit
 1 file changed, 1 insertion(+)
 create mode 100644 bacon.txt
 
ls
bacon.txt
 
echo "bits" >> bacon.txt
git stash
Saved working directory and index state WIP on master: 6d2649a initial commit 
HEAD is now at 6d2649a initial commit
 
git stash list
stash@{0}: WIP on master: 6d2649a initial commit
 
git show stash@{0}
commit d1525