If you're ever lost, pressing L1 will always provide technical details for each upgrade while the shop is open. Press the touchpad, and you'll bring up the in-game shop, which provides all sorts of gear and power-ups. Don't think of it as a permanent mainstay - rather, a temporary fleeting bonus that is reset at the end of each session. So how does the flow of battle work? Completing actions, such as kills or reviving teammates, nets you cash. Make a point to practice this move in several matches so you remember it exists - it'll save your life one day. If you find yourself in a close-quarters situation, or you're sneaking up on a lone foe, the wind-up stealth kill is the easiest way to take them out. New players severely underestimate the power of this ability, as it can one-hit KO just about everyone on the field. One more thing: The grappling hook also has a windup melee attack that you can spring by holding down L1. Case and point: Try hanging out near grappling hook zones, as players can't resist the L1 pop-up, leaving them completely open to fire. Swinging on a rope limits you to the use of a sidearm, and unless you're moving as a group to either outflank an opposing team or you're absolutely sure the coast is clear, grappling too often makes you an easy target.
While it's easy to get overwhelmed by this concept that's tacked onto all of the other nuances that the Uncharted series brings, don't overthink it - in fact, don't even make a point of using it all that often.
You can also use it as a rappelling tool and as a vine of sorts, ensuring a safe fall. By pressing L1 in specific areas, players can toss a line of rope that will initiate a swinging motion, allowing them to leap from setpiece to setpiece. The first new toy you may notice jumping into Uncharted 4's online component is the grappling hook.