The Btrfs hot relocation support comes down to when storing data on a traditional (rotating) hard disk drive, when data gets "hot" (a.k.a. being frequently used), these patches would allow the data to be automatically migrated to a non-rotating disk (i.e. solid-state drive). By moving the frequently used data over to an SSD, the performance would obviously be much more optimal than keeping it on an SSD but making it so not all of your data would need to be stored on a costly SSD.
Published late last year was VFS hot data tracking for the Linux kernel for being able to track the "temperature" of data on the disk to be used in situations like this for auto-migration of data from HDDs to SSDs. An alternative to this in hybrid HDD/SSD environments would also be using the newly-introduced BCache HDD/SSD caching framework that has been added to the Linux 3.10 kernel.
The Btrfs hot data relocation support hasn't been merged into any Btrfs next tree yet nor is it known if it will be merged potentially for the Linux 3.11 kernel, but details on how this relocation support plus the experimental patches can be found on the Linux kernel mailing list.