The internet is a city
In the city there are towns
In the towns are houses and we can think of your website as the house
And then inside that house (your website) there are lots of rooms (the different pages of your site)
Now going back to the towns there are many houses meaning lots of other websites
Now connecting these houses are the roads
And on those roads are the cars (your customer)
And to get to their destination they need to search for the address they are going to
This is the same for SEO, people are searching daily for the answer they need and are looking for the house (your website) that will give them that answer. So in order to get your website showing first we need to ensure your pages use the words they are searching for and this is SEO.