Hi,
this is a long story. VRF is like a virtual/configured router with its own interfaces, routing processes and forwarding tables.
Routing wise in a MPLS VPN environment between PE routers MP-BGP is used and routes are made unique with the help of a route-distinguisher. So two customers with each 10.0.0.0/24 will be seen as different prefixes inside MP-BGP, f.e.
65000:100:10.0.0.0/24
65000:234:10.0.0.0/24
The above noted prefixes are called VPNv4 addresses (96 Bit long).
To setup a rule set where those prefixes should be visible, i.e. in which VRF they should be imported, route-targets are used.
They define VPN membership. This is like a person (VPNv4 prefix) who is member of f.e. Chicago Bears fan club (RT 1) and also member of the Dallas Mavericks fan club (RT 2). A prefix may be found in several VPNs.
Hope this helps a bit. If not, go ahead and ask more detailed questions, you are welcome.
Regards, Martin