In actual MPLS FRR is a feature of RSVP-TE, it is also called MPLS local restoration or MPLS local protection. MPLS FRR gives protection to the LSP path in a network where each LSP is protected by backup path. The node which redirects traffic after path failure to the backup path is known as Point of Local Failure (PLR) and the node where backup LSP merger with primary LSP is called Merge Point (MP). This protection is purely local as compare to protection which is enable at IP Layer (Layer-3) which take a bit more time then this which is not acceptable in real time application (VoIP, Video Conferencing etc). This local protection takes even less than 50 ms.
There are two types of protection approaches:
1. One-to-One Local Protection
In one-to-one approach, PLR maintain a separate path for each LSP across the path or network. This method creates a detour LSP for each protected path at the point of each local failure. For more detail, study RFC 4090
2. Many-to-One Local Protection
In this method, PLR create a single path that can be used to protect multiple LSP. In this method a same tunnel can be used for multiple LSP which acts as a protected path for all links which face failure. See RFC 4090 for more details.
Refer to the figure above, we have Node-A and Node-E as the start and end point for which the primary path (LSP) is from Node-A to Node-E through Node-B and Node-D. While the secondary path for Node-A to reach Node-E is through Node-C. We assume that for primary path FRR is enabled & once it is enable all the remaining nodes on a network came to know about this feature. Assume the link between Node-D and Node-E is down by any means, so the first node who realize this breakdown is Node-D which immediately inform Node-B and Node-A. For Node-A to get the failure message it will take some time and Node-D already know about the link failure & FRR is enable for the LSP so Node-D will use the detour path for the communication (Node-D-C-E) to avoid any loss and get rid of the link failure which will carry the traffic to final destination. This whole process will take less than 50 ms. On the other hand when the backup path (secondary LSP) comes up, traffic will be switched to the secondary LSP and detour path will turn down.
I Hope this will be informative for you :)