Let’s check Google’s SPF record first with dig command.
In the answer section you can see the IP addresses. These are the servers which allowed to send mails via google.com.
So you have your domain name e.g. google.com and you have your mail server on it with an A record mail.google.com. This server can send mails for its own name, but any other servers are not allowed to send mails. With the SPF record, you can send mail from the IP address via google’s mail server. So server 216.73.93.70 and 72 can send mails (relay) via google’s mail server.
Also you can use domain names in SPF record and tell the server to use that instead of the IP address.