Bracing
Bracing, also known as corner braces, end post braces, or gate post braces, are supports that are used on many different types of fencing to strengthen corner, end or gate posts.
Braces can be horizontal, like the one pictured here, or diagonal, often called stays in many parts of the world.
Either way, fence post bracing or stays are designed to help fence posts under tension or carrying weight stay straight and plumb by transferring some of the force on that post to the one alongside it.
There are many ways to construct fence bracing and different materials for different types of fencing. Bracing also isn't installed on every type of fence. Lower chain link fences, for example (4 feet or under especially), usually don't need bracing because there's not a lot of tension on even the corner, end, and gate posts.