A 60 percent keyboard has about 60 percent of the keys of a full-size keyboard. There is no sharp definition which keys must be present, typically a 60 percent keyboard lacks the numpad, the whole navigation cluster (arrow keys and keys above them) and the function row.
Since a 60 percent keyboard has much fewer keys to work with, you will need shortcuts to access many of its functions. For that purpose, there are one or more function keys (Fn, Fn2, etc.) in the bottom row. For example, the function of F2 is often featured as alternative on the key 2, thus it can be accessed by pressing Fn + 2. Note that also the arrow key navigation is usually made accessible via keyboard shortcuts.
It requires some time to learn how to work with a 60 percent keyboard, but it also offers some advantages over a full-size keyboard: A 60 percent keyboard requires less space on the desk and it is easier to transport. Moreover, it makes it possible to have the mouse closer to the keyboard, which is considered advantageous from an ergonomic perspective. Especially a lot of gamers prefer the 60 percent form factor over larger keyboards. The 60 percent gaming keyboard has become a very popular option.