AB=CD is a reversal pattern that helps you identify when the price is about to change direction. The idea is that you can buy when prices are low and about to rise or sell when they are high but about to fail.

The pattern consists of 3 legs, with 2 equal legs labelled AB and CD, together they form a Elliot zig-zag shape. 

This is the simplest harmonic pattern. It is very basic and best to be traded when it is complete (at the Point D). Also, SL should be based on MS not Fibs

Classic ABCD Pattern Example