What SPF is
SPF (Sender Policy Framework) is a TXT record that lists the servers allowed to send mail for your domain. A receiving server compares the connecting IP address with that list. The record starts with v=spf1, contains mechanisms such as include, ip4, a and mx, and ends with an all mechanism that says what to do with everyone else.
-all means reject mail from servers not listed, ~all means accept it but treat it as suspicious, ?all expresses no opinion, and +all allows anyone, which defeats the purpose.