Git submodules are vaguely like externals in subversion - they're references to external git repositories.
What the instructions in wikislurp are saying is that, in order to use the software, you need to not only clone the repository itself, but also clone the repositories that are referred to by the submodules.
The thing you quoted from the README,
git submodule init git submodule update cd externals/curlcall git submodule init git submodule update
are a series of commands to run in order to fetch the repositories referred to by the submodules, which are probably needed for building the software or whatever. Just run them at the same command line prompt you used to clone the repository.
Alternatively, if you have a new enough version of Git, you can abbreviate all of those commands to just git submodule update --init --recursive