In plain words
Your advisers send and receive messages about your money from their own domains. If their email can be forged, a request can appear to come from them.
Why it matters to a firm
A firm’s own settings can be sound while an adviser’s are not. Asking advisers how they confirm payment instructions is a reasonable question.
How anyone can check it
nslookup -type=txt _dmarc.youradviser.ca Run the DMARC lookup on each adviser’s domain. No record, or p=none, means their email can be forged.