The VNI are part of the customer overlay network that you are connecting between VTEP across the data center.
The route targets are part of the underlay transport that is being used to communicate between the VTEP.
This is a pretty good overview on how the overlay traffic works across the DC. Once the underlay and overlay architecture is setup, you can separate tenant in the DC.
http://www.juniper.net/documentation/en_US/learn-about/LA_VXLANinDCs.pdf