Expressions are small scripts that control the value of a specific attribute within Maya. We can use the expression editor to edit and create expressions with our scene.

To open the expression editor, either

a) select an attribute of a node in the channels box, right click, then select expressions.

b) from Maya's menu, select Window->Animation Editors->Expression Editor

 

 

If you select an object, it's attributes will appear in the epression editor. Scroll down to the attribute you wish to control, (in my case the y translation) and select it. Within the text editor portion of the expression editor you can write the expression to control the attribute. In the above example i simply assign the rotation of a locator to the y translation of the sphere.

Once you are happy with your expression you can click "create". If everything went OK, the create button should change to "edit". If not there was a problem with your expression.