In your DMARC record, you have the option to choose one of three options, which are none, quarantine, or reject, respectively. Below we briefly discuss each of them.
p=none simply means that your mail servers will monitor emails coming from your domain, but they won’t block any messages that may be fraudulent. This is a good way to start monitoring scams, but it doesn’t do much to prevent them.
p=quarantine is a way to tell the mail server that you want all emails sent from your domain that have not passed SPF or DKIM checks to be placed in the spam folder instead of the regular inbox.
P=reject goes a step further and tells the recipient’s mail server to reject all emails sent from your domain that have not passed SPF or DKIM checks. This means that such messages will never make it to the inbox (or even spam folder) of the user who receives them.