It's not "I understood ALL" but "I understood EVERYTHING".

ALL must come with a noun or a pronoun, but not alone. If so, we'll change it for EVERYTHING or EVERYONE for people. Look at these examples:

You've eaten ALL THE FOOD! You've eaten EVERYTHING!

Did you understand IT ALL? Did you understand EVERYTHING?

I've done ALL THE EXERCISES. I've done EVERYTHING.

ALL OF THEM are on sale. EVERYTHING is on sale.

I sent THEM ALL a Christmas postcard. I sent a Christmas postcard to EVERYONE.