A multilingual site repeats the same page in several languages. Without hreflang, Google may show the English page to a French searcher or treat the versions as duplicates. The attribute goes in each page's head (link rel=alternate hreflang) or in the sitemap, and lists every version including the page itself, plus an x-default for readers whose language is not served.
The rule only works when each page announces all its siblings and when the targets really exist and are translated. Announcing a French version that is an English page with a French menu is a duplicate signal; SeoGeoMap announces a translation only once the content is translated.