It's very useful if you have multilingual website. Or if the content is the same but some specific elements adapt to the geography of your site users (prices, currency...).The main goal of the hreflang attribute is to ensure that your users are accessing the content that is catered specifically to them
While the main goal is not to increase site traffic instantly, instead the advantages are to advance your site’s quality in the long term and better your search engine optimization. In using hreflang you will decrease your bounce rate( the percentage of visitors that enter your website and then leave without interacting with your page). The site visitors seeing content that is easier to understand, with the correct language or content, will be more likely to stay.
For that same reason, your CTR (click-through rate)and the conversion rate will improve too. The conversion rate is the proportion of visitors who take action- and go beyond a casual website visit. So basically it helps people to do what you want people to do (clicking, reading buying, subscribing...) on your website. And of course your international SEO will benefit from the use of this attribute.
Hreflang is quite easy to grasp. It looks like this:
<link rel="alternate" href="XXX" hreflang="fr-ca"/>
If you have a French-language version of your English-language page you will tag it as french by using « hreflang="fr" ». The first href tag refers to the page itself and then follow the alternatives that matches (this page). For instance if you have an english, a french and a spanish language version of the same page:
<link rel="alternate" href="en" hreflang="link-of-en-version"/>
<link rel="alternate" href="fr" hreflang="link-of-fr-version"/>
<link rel="alternate" href="es" hreflang="link-of-es-version"/>
These tags have to appear on each version of the page. You will paste your tags in each page concerned, by going to the Search Engine Optimization(SEO) and pasting your hreflang in the Additonal tags.
You can also specify the region once you identified the language so it's even more targeted, for example “en-GB”: English content for GB users or “de-ES” for german speakers in Spain. You can't specify a country code by itself!
Now you know the basics of Hreflang, you are ready to translate your website in any language you want and make it known accross the world wide web!