It's always interesting to see someone who understands this concept try to explain it to someone who doesn't.
It's a relatively simple concept, but it's difficult to explain in entirely different terms, without referring to itself.
Since your next question will most likely be, "What is a variable?", check this out..
http://www.rmxp.org/forums/http://dev.r ... 43#p341743
If that clicked, then the switch is now easy. A switch is a variable that can only have 2 values: '0' or '1'.
Which can also be referred to as 'OFF' or 'ON', and 'false' or 'true'. In the case of event commands, they are called 'OFF' or 'ON'.
A switch is used to remember the 'state' of something. For example, you have an event that controls a door (or chest). It has 2 graphics (door open, door closed), and some code to animate the door opening & closing. Each time your player interacts with 'door', it changes from one 'state' to the other. It's either closed or open. So if you associate a switch that state ('OFF' = closed, 'ON' = open), then by looking at that switch you can tell what state the door is in.
There are several tutorials on "chests" & "doors", which are probably the simplest use of switches. Go check them out.
Be Well