Mergeinfo, or more specifically the versioned property svn:mergeinfo, is the history of merges made into a given file or directory. This article gives a detailed look at mergeinfo; what it means and how it works. The svn:mergeinfo property value is simply a newline separated list of merge source paths (relative to the repository root), each source path is followed by a colon, and after the colon a list of revisions previously merged from that source.As always, some gotchas burried here and there:
Note that unlike the familiar -rX:Y notation used by many Subversion subcommands, the version ranges in svn:mergeinfo use an A-B format in which the revision A is inclusive. In other words, if you merge -r3:7 expect to see mergeinfo with range 4-7.
(c) CollabNet Community, Subversion 1.5 Mergeinfo - Understanding the Internals