From Roblox WikiEdit
Tools are weapons or other items held by a player. Tools are represented by small squares on the bottom-left corner of the game screen. They are used to do things like move bricks, fire weapons, fly planes, or just about anything else. Tools can include Guns, Rockets Launchers, Slingshots, Bombs, and more. Tools are programed in LUA and can be modified by users to suit any purpose.
Using tools is very simple. You simply click the tool's icon in the lower left corner of the screen, or use the number indicated on it (e.g. 1,2,3,4,5,6,7,8,9) to select it. Then, you can use the mouse and keyboard to control the tool. For example, the default paintball gun is used by selecting it, and then clicking on the area you wish to shoot at. Press backspace to drop your tool, if it's not a HopperBin.
Reset tools are used to respawn your character if you get stuck. These tools are gradually being replaced by Reset GUIs, but they are still widely used in many games. Reset tools are used by selecting them, and then clicking anywhere in the game area.