A Variable is a value or state that can both be referred to and changed by behaviours. They are used for keeping track of a state throughout the life of the show or scene. They can allow you to perform functions such as creating sequences, counting up the amount of time something has happened, or used to adjust parameters of the show, such as audience size and difficulty rating.
e.g. I have created a Text Variable with the default value of 'mango' in a scene. I want to refer to the default state of this variable in a behaviour so when adding a WHEN condition which refers to this text variable, I would type 'mango' into the field. I also want a different behaviour to change this variable to 'banana', so I would add a DO: Update scene variable action to that behaviour and type 'banana' in the text field. Another behaviour could refer to this 'banana' state, or change it to 'apple' and so on.