The ping latency (or just ping) is found on the very top right hand corner of the screen. It shows a number that changes higher and lower. The ping determines the time it takes for an input from any key, button, or simple mouse click to cause an output in your game.
A high ping is generally not a very good thing. A high ping can be caused by many things, but it's mainly either a bad internet connection, or you are in a server you shouldn't be in to play well (example: you live in America but are playing in an Asian server). This causes what many call "input lag" where you press a key and it takes a long time to respond. This means the input is taking long to cause an output. Say i press the space bar (jump) and i have a large ping. Once i press the space bar, it will take long to cause a jump animation in game.
A low ping is a great thing. It allows you to play better, and see the game more smoothly. It causes less lag and almost no input lag. The reaction times are on point, and everything is easy to work with. When you press a button, the game should automatically respond. To get a generally low ping from the get-go, simple play in the server of the country you live in. (example: an American playing in an American server would cause less lag than others).