In brief
A Git submodule connects a separate repository at a particular commit to the parent project. It does not automatically follow every latest change on an external branch.
Useful to know
A Git submodule connects a separate repository at a particular commit to the parent project. It does not automatically follow every latest change on an external branch. [1]
After cloning, check that submodules are initialized at the recorded revisions. An edit inside the submodule and an updated reference in the parent are separate changes. Review both when updating the dependency.
Sources
For further detail and to check the information: