./ tells the shell that you are referring to a file in the current directory. If you wanted to call a program such as
backup and this happened to be a binary available in one of the
program directories on the system, the shell would by default run this binary.
However you might have been in a directory of your own programs which happened to have a binary called
backup, too. You might hence not even realise that the system was actually running a completely different binary.
./backup, you are specifying that you want the system to use the binary in the current working directory, as opposed to one available in the program dirs.
You can test this by using the Linux
whereis command, which in this example might specify that
backup is actually at
/usr/bin/backup, instead of perhaps