Overwatch developers have released a video where they have tried to explain how the higher tick rates would affect the average Overwatch gamer. The game when originally released was 21 Hz tick rate. The low FPS meant that there were performance issues in the game. While it was not that far off from other FPS Shooter games, many believed it could be much higher.
Blizzard has been rolling out 63 hz tick rate updates since August 2016. This update has slowly rolled out in phases to various regions. By now, all regions should be affected by the update.
Check out the video to understand the technicaliities of the update.