when barry travels back in time in episode 23 to save his mother, there is also his future self and young barry. so when the future barry saw present barry, you can see that he signals to him not to save his mother and why is that? my theory is that flashpoint already exist and we all know the effects of flashpoint. the events that take place when barry went back in time to save his mother. what if the events of flashpoint has taken place and this time future barry went back in time again to stop himself back then which is the present barry from saving Nora Allen so as to avert the flashpoint events. that may be the reason why future barry signals to present barry not to save his mother because future barry knows the outcome if his mother is saved. so by stopping present barry from saving his mother, he avert flashpoint and repeat the timeline by saving yet again his younger self so that he will still one day become the flash cause originally barry becomes the flash not by the hands of reverse flash but by accident itself.