What SPF is
An SPF record is a line in your domain's DNS listing the servers allowed to send email as you. Receiving inboxes use it to spot forgeries.
Why it matters
No SPF — or a broken one — and your mail is far more likely to be filtered as spam, or spoofed by others.
Honest note: A valid SPF record is only one signal. It does not by itself guarantee inbox placement — DKIM and DMARC matter too, as do content and reputation.