Can anyone explain me how´то' works?

I'm sorry if it's too basic or repeating question. But I couldn't find it in my grammar book. Not sure why they dropped it.

So I don't even know what to call it but I'm asking about 'то' when it's used like ´Я сделал (то), что должен был'

What's the difference between that sentence and ´Я сделал что должен был'?