Stash is a web application that hosts your git repositories for you – you might think of it as a git “server”. SourceTree is a GUI “client” that allows developers to work with git repositories on their local computers. SouceTree has the ability to clone repositories from your Stash server (or any other git server).
Usually you would host one instance of Stash and grant all of your developers read and write access to some projects and repositories hosted on it. Developers would then download and install SourceTree on to their development machines themselves and use it to work with their own local copies, or “clones”, of the repositories hosted on Stash.
Hope this clears it up for you!