In general, if you are cloning a git repository, then it is most likely that you are downloading source code, which you will need to compile before you can install and use it. For many packages, this will mean running the following commands:
./configure
make
make install
It is recommended to run the first two commands as an unprivileged user and the last one as the superuser (root).
There are also some useful guides on the web for compiling software, such as this one.
However, that package you link to looks very simple and it doesn't seem to follow those general instructions above. It looks like just running ./ARP_Spoofing (possibly with one of the listed options) from the source or installation folder should work. There is also a simple makefile in there, so if you wanted to re-compile it, it looks like you can just run:
make clean
make