The term “hosting” doesn't describe one service, but a variety of services which offer various functions to a domain name. Having a site and emails, for example, are two individual services although in the general case they come together, so many people consider them as one single service. In reality, every domain has a couple of DNS records called A and MX, which show the server that manages each specific service - the former is a numeric IP address, that specifies where the website for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that manages the e-mails for the domain name. For instance, an A record would be 18.104.22.168 and an MX record can be mx1.domain.com. Whenever you open a website or send an email, the global DNS servers are contacted to check the name servers that a domain name has and the traffic/message is first directed to that company. When you have custom records on their end, the browser request or the email will be directed to the correct server. The concept behind employing separate records is that the two services use different web protocols and you can have your site hosted by one service provider and the emails by another.