Your registrar is the company where you registered your domain name. This could be any hosting or registrar company such as GoDaddy, Network Solutions, Register.com, or many, many other companies.
You can choose to register your domain name for any number of years. If you do not re-register your domain name, you lose it and someone else can purchase it. A domain name can only be registered by one entity, just like a phone number or a physical address. It is unique, and the registrar has to make sure when you are registering your domain name that no one else has this domain name already.
We are going to relate all of these items to a physical coffee shop:
If you think of a coffee shop, the registrar would be the Secretary of State in the state where you registered the name of your coffee shop. You have to file an annual report in that state, and they have on record the name of your coffee shop.
That is a Registrar.
Sometimes your registrar, name server and host are all the same company, so one company does three functions. Sometimes they are all different so you would have three companies doing three functions, or maybe two companies do those three functions.
There are different ways to find out who your registrar is. If you go to a website such as https://whois.domaintools.com, you can type in your domain name and you will be returned with the registrar for your domain.