Google Sites is a website building platform from Google. If you're familiar with other website platforms like WordPress or Wix, you can think of Google Sites as something that's somewhat similar, but perhaps more specialized for businesses, web-based teams and education.
Google Sites allows you to create a website without having to know how to code it yourself. It falls under the Collaborative category in G Suite, meaning that you can get other Google users in on the website creation process too, which is what makes it so powerful and such a valuable tool for teams.