When your run apt search it will provide you with a list of available
packages from your configured repositories. If one is already installed on
your system, you should be able to see it with a ‘[installed]’ notice on
the right hand side of a package in the list.
Regarding installation, it means that you execute a package that will add
appropriate files to your system so the software can run when called. Most of
the time, you’ll do it from your repositories and so, if you want to use
the command line, with sudo apt install You need to use sudo because you’re
installing a software.
For more information, I recommend you read the Trisquel’s documentation
which details the various methods to add and remove a software from your
system.
