Is this a stress fracture or natural cedar characteristics on a swing set?
This is typical of wood movement. The wood will eventually start to rot because of the "pockets" created by the cracks. Those will hold water, expediting the rot that will occur over the years.

Cedar is rot resistant, but with these cracks it will quicken the inevitable. If a well formed metal cap can be added to the length of the top which would be preferred so it can be fastened on the sides, or covered with a peel and stick waterproofing, so the movement of the wood will not affect the covering. This is another preferred fix, since a metal cap is a nice harbor for insects, depending on how close it is set to the wood.