I don´t understans this about magic resistance.
How does it work???
Is it a dice or what???
Is for exampel, 10% M.R = 10% to fail spell on person???
Please help me to understand.

MR goes from 0 to 100. You roll 2 10-sided dice, 10s and 1s. That prodecues a percentage for you "resistance save". If you succede, the spell has no effect on you. If it fails, you then may make a saving throw if applicable.

If you have a MR of 25% and someone casts magic missile against you, you roll the two ten-sided dies. If you roll a 2 and a 7, that would be 27% and you would fail your magic resistance. Then you get to save - buth there is no saving throw against magic missiles. If you rolled a 2 and a 1, you would get 21% and would therefore be immune to the missiles and would take no damage.