The ‘hreflang’ tag tells Google and others which audience each page is meant for. Here’s a good look at how to use this tag.
When a site has been set up to serve multiple countries or languages, search engines can still struggle to decide which version of a page to show. That’s where the ‘hreflang’ tag comes in. It tells Google and others which audience each page is meant for, so everyone is provided the version that fits their language, currency, and expectations.
A good, up to date look at how to use this often-misunderstood tag is provided by Ask An SEO: What Are The Most Common Hreflang Mistakes & How Do I Audit Them? on Search Engine Journal. It’s worth a read.