With the increase in Internet users, there is too much content on the Internet. Way too much to pick! And Search Engines are playing a very important role helping the users to find exactly what they want.
So it grew into a set of rules and methodologies which are needed to be followed.
These rules will give proper information about your content on the Internet to the Search Engines.
And then the search engines use this information to know what your content is about, sending the users to your content directly.
Therefore, optimizing your website/content for the search engines so that they can find you easily, is called Search Engine Optimisation.