They are like Windows’ shortcuts, but in Linux. They’re an alias to other file. The way of using them is

ln -s <path/to/file> <path/to/linked/file>

Maybe it’s easier with a more concrete example.

Imagine I want to have my repositories folder which is on my home directory in my desktop, I could copy and paste them but that would be terrible.

Why not better use a symlink. I could do

ln -s ~/repositories ~/Desktop

Now, on my desktop there will be a folder called repositories, and when I open it with my file manager I’d be seeing the contents under ~/repositories.