Search engine registration is the largely outdated practice of manually submitting a website's address to a search engine so it knows the site exists. In the early web this was a necessary first step; today, discovery happens automatically through crawling and links.
Modern crawlers find a new site the moment a single external link points to it. If your page is genuinely on the web and reachable, Google and Bing will discover it without a submission form — and many old "submit your URL to 500 engines" services were low-value at best and spammy at worst. The concept survives only in a modernised form.
These steps are standard parts of technical SEO setup and replace anything the old registration ritual once did.