When it comes to marketing, many ― likely most ― companies are going digital.

“Businesses want to meet customers where they are, and that’s often online,” says Margaret Lilani, vice president of talent solutions at freelancer platform Upwork. As a result, she says, for anyone looking for freelance work or a side hustle, digital marketing skills “are going to be highly sought after.”

These include social media marketing, email marketing and lead generation. They also include search engine optimization, or SEO. Experts on Upwork offering this type of marketing charge as much as $125 per hour.

Here’s what SEO is, exactly, and how to get started.

Search engine optimization is the process of ensuring your website gets picked up in organic searches, say, if someone is looking for the kind of products you sell or the kind of content you cover. Imagine doing a search for French fries, snowboarding or Donna Summer on Google. SEO helps sites show up closer to the top of the list Google shows you that aren’t ads.

Experts in search engines like Google and Bing study tactics to ensure the platform considers your site a go-to on whatever you’re offering. They’ll make sure it includes relevant keywords, links and content, among other tactics.

“Search engine optimization is paramount if you want to be found when people are doing online searches,” says Yolanda Owens, career expert at The Muse, adding that it’s “increasingly becoming more and more valuable.” And employers are on the hunt for experts in the field.

A search for SEO jobs on ZipRecruiter results in more than 14,000 open roles, both part and full-time.

How does one become an expert in SEO? There are various courses and certifications available online, including from search engines like Google itself. Colleges and universities offer courses as well. These range from free to as much as $1,000 each, so do your research before signing up to make sure you’re taking the best course for you.

Once you’ve learned the basics, the best way to build that knowledge base is by getting some on-the-ground experience. Create profiles on sites like Fiverr and Upwork and start picking up gigs on sites like ZipRecruiter, LinkedIn and Monster. The more experience you accrue, the more valuable you’ll become as an expert and the more you’ll ultimately be able to charge.

As long as search engines are a key way in which people find what they’re looking for, the skill likely won’t go away. “You spend all this time and all this money building your site,” says Lilani of companies’ attitudes, “if nobody can find it, again, why did you do that?”