This service stores a snapshot of a webpage so that it is possible to cite it for reference purposes, even if the original page disappears.
The primary user of this service is intended to be Wikipedia and its sister projects that heavily rely on being able to keep references online for years, often long after the original web page has changed or been taken offline.
archive.grok.se provides a cryptographic checksum of the content, so that the link does to a provably specific version of the page. It is highly unlikely that someone would manage to alter the contents of an archived link in such a way that it is not detectable.
Any questions of comments should go to User:Henrik on the English Wikipedia.