A drawer is a container for other controls that is generally not visible and can be activated by sliding one edge of a touchscreen from the outside to the inside of the screen. This triggers a gesture that will bring the drawer (now visible) in the current view by following the user's finger on the screen. It is a very common UI element as it adds some available space for UI controls without losing focus on the main content of the current view and without permanently stealing space for other controls. A typical usage is for menus or navigational items that the user can use to change/tweak the current view's content or switch to another view of your application.
Apart from a gesture, it is usually associated with a button you can press to trigger an animation mimicking the gesture behavior. It is not mandatory, but most people will look for a button with the hamburger icon (three horizontal lines stacked up, or three bullets stacked up) that is now universally...