A data center is place where are located a lot of servers.
Data centers represent the heart of the organization of search engines. When you do a search on a search engine, it is often the datacenter closest to your home that takes care of providing you the appropriate results to your query. If the closest datacenter is busy, another datacenter will replace it to answer you.
The data centers are spread all over the world and are made up of tens of thousands of servers to deal with the incredible volume of researches done every day on the planet.