A large distribution grid can work like this. There're several power station each outputting 50 Hz AC. Each power station feeds energy into a substation next to it which raises the voltage and then feeds energy into a powerline and that powerline goes to a substation close to the customers. The deal is customers don't really want to depend on a single station and that single powerline so they've crafted a distribution ring - there's a chain of high voltage substations connected to each other and each power station feeds energy into that ring - each substation in the ring is connected to two of its neighbors and also to the power station. The more advanced design is to have each power station connected to two of those substations via a separate powerline.
Now electricity "moves" at the speed of light which is high but still finite and over the distance of dozens and even hundreds of kilometers the phase difference between the different power stations will be notable and because of that phase difference different power stations should partially cancel each other out.
If several power stations were connected to a single point they could have been synchronized appropriately but in the described setup there's no single point - there're multiple substations separated by long powerlines forming a closed ring and so it looks like something will always be out of phase with something else.
How is synchronization possible in such conditions?