svnsync is the Subversion remote repository mirroring tool. Put simply, it allows you to replay the revisions of one repository into another one.
In any mirroring scenario, there are two repositories: the source repository, and the mirror (or “sink”) repository. The source repository is the repository from which svnsync pulls revisions. The mirror repository is the destination for the revisions pulled from the source repository. Each of the repositories may be local or remote—they are only ever addressed by their URLs.
The svnsync process requires only read access to the source repository; it never attempts to modify it. But obviously, svnsync requires both read and write access to the mirror repository.
svnsync is very sensitive to changes made in the mirror repository that weren't made as part of a mirroring operation. To prevent this from happening, it's best if the svnsync process is the only process permitted to modify the mirror repository.