A symbolic link is a file pointing to another file.
For example, you can create a symbolic link in your home directory that points to a directory on your external drive.
To make a symlink, go to the main menu: File –> Make Symlink:
By pressing ⌥⌘L on the keyboard you can create an alias:
An alias is a small file that represents another object in a file system (e.g. remote system) and provides a dynamic link to it. The target object may be moved or renamed, and the alias will still link to it.