Concept of Realities
According to MCU(Marvels Cinametic Universe)
As Dr. Bruce Banner said in Avengers Endgame that Any change in past does not affect the present in which we are,it will create a new reality.
It simply means you travel to past and corrected some mistake you have done in past. Then it will not affect the present in which you are or in that present from where you have gone to correct past mistake. This will create another present or reality in which you don’t have done that mistake.
If you have any doubt let me know in comment section.