The Novikov Self-Consistency Principle is a theoretical concept in physics which states that any events that occur in time are subject to the constraint that the timeline must remain self-consistent.
Or, as the great physicist Igor Novikov himself put it: "If a man were to travel back in time to a point before his birth, he would still not be able to kill his mother, because she would already be dead."
Or, in other words, the universe will always find a way to make sure that the past and the present are consistent with each other, no matter what you do.
But don't worry, it's not like you can actually travel back in time and kill your grandfather or anything... or can you?
The Time Traveler's Dilemma