When in the VAB and Spaceplane Hangar, there is a tab in the upper toolbar, between the "crew" and "parts" tabs (look for the blue button that has a Kerbal on it), used for what are called "action groups". Some parts are automatically sorted into these groups, such as lights, or wheel brakes. There are also custom ones configurable to the keys 0-9, where when you press the corresponding key, will activate the option(s) you selected of the part(s) you have placed in the group. Edit: these actions activate in-flight, not in the editor! While in the action groups menu, you cannot place, move, or delete parts. Clicking on a part will bring up its options, if it has any. These include decoupling, undocking, toggling several features, or deploying/undeploying. With the desired group selected on the left by clicking on it, subsequently clicking on an available action will bind it to the group. Clicking on an action in the group will unbind it.