An SPF record is a hack. It's a way of using the DNS protocol to say who is allowed to authoritatively send email as a domain name. It does this by using a DNS TXT record (as opposed to an A or MX record). TXT can contain any text. An SPF record is a specially encoded TXT record like this:
"v=spf1 ip4:198.51.100.26 -all"
The v=spf1 indicates that this is an SPF record, considered version 1 of the spf standard.
After the version, there are any number of 'directives'. In this example, there's only 1...