An optimized website means that it is structured, coded, designed, and has content published in a way that will allow Google to rank it.
Why is this important? Well, if Google can read your website correctly and interpret your content then it will understand the product or service that you are selling and so present it to your potential customers searching on the Web.
So, how do you ensure your website is optimized? Certain basics need completing such as keyword research, great content, and some technical criteria that a consultant can help you with such as tags, architecture, crawlability, among many other things.