By working together, they airlines can offer a large number of flights to lots more
Destinations. This is called code-sharing.
KLM can sell flights to destinations served by Air France and Delta Airlines with their own flight number, but passengers will not be flying on KLM.
For example, KLM doesn’t fly to Phoenix, Arizona, instead a passenger will fly to Los Angeles and connect onto a Delta flight and fly on to Phoenix. This flight will be sold as KL7762 and won’t be a KLM aircraft but a Delta plane.