This profile enables your site to use the FOAF semantic insertions for richer metadata on your site.
FOAF goes for Friend Of A Friend and aims to interconnect in “knowledge nodes” all the personal interconnections.
Despite it started as a Person to Person oriented RDF vocabulary, it went beyond and included some different larger entities where any PERSON could fit in, like a number of individuals would fit into the same ORGANIZATION.
It was quite under the radar until it was de-facto included into the Schema standard promoted by Google, Bing and some other big players in Search.
Theoretically you need to add this profile line in order to correctly use FOAF elements in your site.
These elements are indeed recognized and properly named in Google’s Structured Data testing tool.
Take a breath and go FOAF Vocabulary Specs
From my personal experience, I can tell you needn’t to use the profile line in order to have your metadata in use, but I do use the line as expected.