Image you have a load that have a certain resistance (you are to decide it's value).

Then ask yourself this question:

- Will the voltage over the load be the same in both cases?

- Will the current through the load be the same in both cases (?)

- If the terminals is shorted, will there flow the same amount of current in both cases

- If you remove the load and let the termnals open, will there be the same voltage in both cases.

If yes is the answer on all those questions, then the circuits are equivalent.

Normally, the two first questions can be omitted, but by answering those - espechially if you take your time to actually calculate on a couple of examples - that should prove the concept for you.