What is the pool in Nicehash?

I am new to Bitcoin and recently have invested in bitcoins. I came across Nicehash when I was planning to get into Bitcoin mining. I understand that Nicehash gives you an option to rent hash power for mining. But what I don’t understand is the pool option in it. What exactly are the pools in Nicehash and how do I create a pool account or join a pool or something? I don’t want to spend any money on Nicehash without understanding what exactly it is used for.