Read here to learn more about git stash operations: https://git-scm.com/book/en/v1/Git-Tools-Stashing

A mercurial equivalent of Stash is called Shelve: https://www.mercurial-scm.org/wiki/ShelveExtension

• If a mercurial repository is opened in SourceTree, the function will appear as Shelve instead of git's Stash.

To stash your changes:

1. First, there has to be a change done to a file.
2. Once there's a change, then the Stash button at the dashboard will be enabled by SourceTree to be used:

Clicking on Stash will trigger this window to pop up:

Steps:

1. Enter a description message
2. Check "Keep staged changes" if you want to keep the rest of the changes that are staged.
3. Click OK.

You will then notice a new column that appears at the sidebar, called Stashes. And the file is now changed back to the state before it was modified:

To apply back the changes, simply right-click on the Stash created, and click Apply Stash. Your file should revert back to the modified version: