Gamemode allows you to alter what you’re able to do, there are currently 4 game modes in Minecraft.
To change your game mode you need to have the OP status for the server. If you need to know how to give yourself OP, see this page.
Setting your game mode is simple, you only need to type /gamemode (chosen gamemode), the (chosen game mode) can be replaced by one of the 4 options listed above.
You can also make this change from your dashboard under the Settings tab.