Headless CMS? Confused Newbie

So I am starting from zero other than some basic HTML/CSS/JS experience. I have seen people online advise that I design and create a Wordpress site about something I have an interest in and spend time building that and optimizing the SEO. But I also heard headless CMS systems are the future for most client websites going forward.

I am confused, can someone clarify this and their place in the web ecosystem? Why do so many freelancers, agencies and clients use Wordpress if a "Headless CMS" is superior for your typical site?