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.
What this check looks for
A domain may publish exactly one SPF record. Two records cause a permanent error at the receiver, and the usual cause is adding a second record for a new mail service instead of merging it into the first. The check also flags a record that ends in +all, has no all or redirect at all, or appears to need more than the ten DNS lookups the standard allows.
The lookup count is a count of the mechanisms in your own record. Nested includes inside other providers' records add to the real total, so a record that is close to ten here may already be over the limit.
What SPF cannot do
SPF checks the envelope sender, not the From address people see, and it breaks when mail is forwarded. On its own it does not stop spoofing of your visible address. That is what DMARC is for, together with DKIM. A valid SPF record also does not guarantee that your mail reaches the inbox.