lost+found is used by the filesystem check (fsck). fsck looks for inodes (areas on the harddisk) that are marked as used (i.e., not in the free list) but do not have a directory entry pointing to them (i.e., these areas on the harddisk are not marked as free, but there is no file that is using them--which is an error). fsck will ask if you wish to reconnect these; if you say yes, it will try to create a file entry in the /lost+found directory on that filesystem.
So it is used to recover data after a filesystem error occurred; for example during power outage or such. With ext4 as default filesystem, this is occurring very infrequently (I haven't seen it ever). It is locked because recovery of lost+found data should be done by the administrator (root user), and not by normal users.
fsck is run automatically every so many boots (you have probably seen it during start-up).
Linux Mint 14 Nadia / 64-bit / Cinnamon