SPF is a method of identifying the server(s) approved for sending email for a domain. You publish it as a TXT record in the DNS of the domain in question (and note that there's a mailing list for help, which the record you supply needs ;>).
As Teddy said, SPF isn't perfect and it has problems if you implement it without knowing what you're doing. That doesn't (in my book) make it stupid, it just means that it's easy to shoot yourself in the foot if you rush at it. Personally I've been using, and checking, SPF for a couple of years now with no problems.
It also only addresses half the problem, you'll probably want to look at DKIM (not Domain-Keys) too.