I would like to know how i could make it where a player will randomly recieve a crate key while he was killing mobs.
Well If it was the point of this thread, then a plugin is something for a Minecraft server to change certain aspects, whether in-game or in the code.
If you were willing to code, you could use a listener for an attack event then check if the player is attacking a mob. Then, you could dispatch a command to give them a key.
I can make you a skript plugin for free to do it with http://www.spigotmc.org/resources/mystery-crate-crate-reloaded.861/ PM me if you want me to with the details