Another word for series is inline. This means you will cut the wire in half and connect one lead of your new item to one of the wires and the other lead to the other wire. This puts the new device "INLINE" with the current wire. This is usually done with the positive (12 volt) side.
Think of it like putting a water filter inline (in series) with a hose. You cut the hose and connect one end of the filter to each end of the hose you just cut.
Your dimmer has 2 input and 2 output wires. It should have come with a wiring diagram showing how to connect it. I once looked into putting one of those dimmers in my rig but ended up just turning off the main ceiling lights and using the 2 lights in the dinette area when less light was needed. Call me lazy.