In plain words
Without it, anyone can send email that says it is from your firm, and most inboxes will accept it. At p=none mail servers are asked to take no action, so it reads reports but protects nothing.
Why it matters to a firm
Forged invoices and changed payment details often arrive from an address that looks exactly like yours. A DMARC policy at reject asks mail servers to refuse mail forged in your exact name. It does not stop a look-alike domain.
The check behind the finding
nslookup -type=txt _dmarc.yourfirm.caIf nothing comes back, there is no policy. If it says p=none, forged mail is still delivered.