A symlink is a file that contains a shortcut (text string that is automatically interpreted and followed by the operating system) reference to another file or directory in the system. It's more or less like Windows shortcut.

If a symlink is deleted, its target remains unaffected. If the target is deleted, symlink still continues to point to non-existing file/directory. Symlinks can point to files or directories regardless of volume/partition (hardlinks can't).

Different programs treat symlinks differently. Majority of software just treats them transparently and acts like they're operating on a "real" file or directory. Sometimes this has unexpected results when they try to determine what the previous or current directory is.

There's also software that needs to be "symlink aware" (like shells) and identify and manipulate them directly.

You can upload a symlink to Dropbox/Gdrive etc and it'll appear as a normal file (probably just very small filesize), but it loses the ability to act like a shortcut, this is sometimes annoying if you use a cloud service for backups as it can create filename conflicts and you need to make sure it's preserved as "symlink" when restored. Most backup software is "symlink aware".