hreflang is a signal that tells an engine which language and region a page is meant for, so it can show the right version to the right person. If you have the same page in British English, American English and French, hreflang links them as a set and says which is which.
You declare it with link tags in the head, or in the sitemap, one entry per version, each naming a language and optionally a region: en-gb, en-us, fr. Every version in the set should point at every other version, including itself, and the pointing must be mutual, or engines ignore it.
Get it wrong and engines fall back to guessing, which can mean a French reader gets the English page, or two near identical language versions compete with each other in search.
You only need this if you genuinely serve different languages or regions. For a single language site it is nothing to worry about. Topkay flags missing or non mutual hreflang when a page declares it, and does not push it on sites that do not need it.