A Composite chart is one created using the Mid-points between each planet in your chart and someone else's.
This is useful to give you a better understanding of the relationship that the two of you have created.
Unlike a Synastry chart which uses both sets of data and compares them, a Composite chart creates a whole new chart: One that represents your combined forces.
If you're in a business partnership I definitely recommend making a Composite chart!
You will need to know how your combined forces are going to react in your business.