It concerns Time Dilation... a consequence of Einstein's Special Theory of Relativity. The faster you travel the slower time passes for you compared to someone left at home. You could travel very quickly for a few (of your) years and return to find your children are older than you are. (Perhaps grandparents? )
For this you must have time traveled,killed your grandfather, before he would have produced any children. If that is possible, then there is no possibility of you becoming his grandson.
In relativity it is assumed that if you travel at the speed of light for a while, you may travel back in time and see your grandfater as a kid. In fact the life goes faster than you, you don't travel back.
