A Fuse is a device which sacrifices itself ( gets destroyed and stops letting electricity through ) if the current passing through it is higher than a certain value.

As such, adding a fuse between your power supply and a power input on your Smoothieboard protects you against short circuits, overloading, mismatched loads, or any kind of device failure.

You need to choose a fuse with a value higher than your "mormal" current for a given circuit. For example if your heated bed consumes 10A, you want to have a 15A fuse protecting it, that way if everything is fine the fuse does not burn, but in case of a short circuit, it does.

Here is an example of a fuse protecting the mosfet power input :