SourceTree, as a graphical front-end for git, is a distributed version control system, so the files that you are editing are local files stored only on your own workstation. There is no need or capability to lock them. If another developer is making changes to the same file, they will need to be merged, either (ideally) through a merge operation if you are working on distinct branches or when one of you does a pull to update your local repo.
"Checkout" in git has a very different meaning than it does in centralized version control systems. In git, "checkout" means to select a commit or branch and set your working directory to that version.
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