Then your page probably won't be listed on their catalog. Registration is usually free but does require some effort on the part of the page creator. You, the creator of the page, must log on to the search engine and fill out a form that provides the details required by that particular search tool. It also helps if your page design includes certain features that are used by the search tool's automatic mechanisms to keep their catalog updated. It also helps if your page is not static.
Many search engines drop sites that are inactive.
Keeping your web site visible then can become a full time job. There are thousands of search engines out there. Their rules are all different. Some are specialty tools and hone a search to a particular type of material. Others categorize web sites and require understanding of their catalog structure.