My light switch has five wires?
You have correctly identified supply, and ground.  

Now look closely at the legend (labeling) on the switch handle.  See where it says "off" and if you flip it, "on"? No, you don't see that? That's because

It's a 3-way switch

The dead giveaway is the black screw on the supply being unlike the brass screw also in sight. 

The other two wires (the brass screws) are travelers. When you throw the switch, it alternates which one is connected. As such, they are interchangeable, and I like to mark them both with the same color of tape, e.g. Yellow.  In my world, two wires in a switch circuit, taped the same odd color, are always 3-way travelers. 

There will be another 3-way switch somewhere, also with those same two wire colors going to its traveler terminals. 

3-ways are complicated to add dimmers.  In some locations Code requires both locations can turn on the light for safety reasons, and that fails if the dimmer at the far location is set to "very dim".

We would need to know more about your circuit and location to give further advice.  This won't be the drop-in you were hoping for.