SEO, Search Engine Optimisation, regards the the service of optimising your webpage content for search engine to relate better to query given.
Take for eg. You need apples, so you go to supermarket, buy the fresh good quality apples.
Here, apple is query, supermtket is search engine, apple bought are search results.
As a SEO person it is our job to organise your content so it is recognised by search engine and match the right page to the search query keywords/phrases.