To add to what other users have said, search engine optimization (or SEO) is done to try to get one's website or other content to appear higher in search rankings.
For example, if you have a website (or YouTube channel) about cooking or video games, you'd most likely want it to appear as one of the first results when someone types a related term into a search engine, like Google, Yahoo!, Bing, etc. By having a link to your website, channel, or content higher in the search listings, it would thus receive more traffic than items that rank lower than it.
There are a variety of techniques that can be used, depending on the search engine, to make one's content appear higher in search listings, which people often refer to as SEO. This can involve multiple factors, such as page titles, keywords, meta data, links, etc. and is often something that people try to improve upon when they can, as it can help a website to gain more visitors.
Depending on the individual search engine (or even site searches), the algorithms that determine which content appears first may differ, but in general it's good to try and ensure that a website is as optimized as possible for appearing in search listings.
For some more information, I'd recommend that you do a quick Google search for search engine optimization, or SEO. There is lots of information available for free, and there are even companies out there that will assist with SEO, should one wish to pay for it. However, you can probably do a fair bit of SEO on your own, without the need for paying anyone.