brent pip is a python package manager. AFAIK it's pretty common to just refer to pip install [name of package] for installation instructions when a python package is available on pip. But I am happy to explain more detail (because that command verbatim won't work for you on Solus):
If you pip --version on Solus you will see that it mentions it is for Python 2 and not Python 3. So you need to pip3 install stimer. (Installation instructions often just say pip instead of pip3 because on other distros pip might refoer to Python 3's pip). So pip3 install stimer should work, but it will install to your user home directory if you don't prefix the command with sudo and you will see a warning letting you know you need to add the user directory (can't remember exactly what the directory is) to your path if you want the package command (stimer in this case) to work from anywhere in your terminal. You can do that, or you can install it with administrator rights sudo pip3 install stimer
So in short, sudo pip3 install stimer should work then you can use the program as shown in "usage".
