A switch is an electronic component that can turn on a circuit, interrupt current, or have it flow to other circuits. The most common switches are manually operated switches with one or more electronic contacts that allow people to operate the devices or simply give commands. The "close" of the contact means that the electronic contact is turned on, allowing current to flow while "open" refers to contact not conducting to form an open circuit where current cannot flow.
The simplest type of switch has two pieces of metal that act as contacts. When the two contacts reach each other, the current forms a loop. Otherwise, the current is open. There are also switches using movable parts to make the contacts conductive or non-conductive. This type includes a toggle switch, push-button switch, rocker switch, rotary switch, etc. Other types of switch might be non-mechanical ones such as capacitive switch, resistive switch, piezo switch, optical interrupt switch, reed switch, Hall switch, etc.