Your site is visited by spiders or bots.
Basically, it is an automatic visit to your site from a specific software (robot or spider).
This bot will gather all the specific information from your site and index it in a mega directory!
The Google search index for instance already contains billions of pages.
This number keeps growing as new websites are created.
After a few hours, days or months depending on the size of your site and its popularity, this spider comes back and checks for changes and new pages since their last visit.
All this happens automatically while you sleep ;)
That’s one of the great blessings of being online!
You have so many automated processes that help you promote your site!
This whole process is called site indexing.