Gear is any number of Roblox created items that can be used in-game such as a weapon or mode of transportation. For more information on gear, including how to tell what types are allowed in a game, please click here.
When a game loads, it will automatically use gear you own of the allowed gear-types to fill the inventory bar at the bottom of the screen. Each of the ten inventory slots will list a hotkey in its upper-left corner indicating what number will need to be pressed on your keyboard to equip the item for use. Basic gear functions can be activated by clicking with your left mouse button, though some gear may have special functions that can generally be found in the description on the item's details page.
To adjust which gears are assigned to specific hotkeys or to use other allowed items that weren't automatically added to the inventory bar, click on the backpack icon located in the upper-left corner of the screen:
This will open up your backpack and allow you to rearrange your inventory. You can do this by clicking on an item and dragging it to a new position. To exit this mode, click the backpack icon again.
For more information on gameplay and building related matters, see the following links: