Route Targets are applied or exported to prefixes and then injected into MP-BGP as an additional Path Attribute. This allows other PE routers participating in the same VRF to import the specific routes out of BGP by defining the Route Target in the VRF import configuration.
This also allows for overlapping MPLS VPN routes where some complex VPN configurations import route targets from OTHER VRFs in order to learn routes.
Each VRF needs to import/export at least one RT, typically for a single customer a single RT value is shared among all PEs participating in that customers VRF, this allows for only routes injected into BGP with the appropriate RT to be pulled from BGP into the appropriate VRF at all other sites.
The act of importing routes from other customer VRFs is referred to as route leaking prefixes between VRFs.