Optimize means to make the best possible use of something. If you optimize your home storage capacity, you clean and organize your closets and drawers to be able to fit the most in.
Before the advent of the Internet, the word optimize was mostly used to describe the act of being really positive about the world, or being an optimist. But today one can scarcely hear the word without thinking about how to optimize a website or make it as efficient as possible. SEO, standing for "search engine optimization," refers to this process of optimizing a website to make it appear more frequently in web searches.