DMARC has 3 policies:
none
reject
quarantine
This policy is configured in your DMARC record. It looks like this:
p=reject;
It's easy to change your policy. Just change p=none; to p=reject; for example.
The policy instructs mail servers what to do when they receive email with your domain. The next few chapters will explain each policy.
None
The none policy does not interfere with your email traffic. Basically, it is a neutral setting and let's email servers decide by theirselves what to do with invalid email. This policy is used often when you configure DMARC for the first time. It allows you to monitor the email flows and make adjustments before quarantining or rejecting emails.
Quarantine
The quarantine policy instructs email servers to be extra careful when the validation mechanisms fail. This may have the following outcomes:
Invalid emails get placed in quarantine where an administrator will check each email manually.
Invalid emails get delivered in the spamfolder of the receipent.
Aggressive spam filters may block invalid emails.
Reject
The reject policy instructs email servers to block any invalid email. The receipent will never receive or get to know the email as it will never get delivered in quarantine, the mailbox or spamfolder.
You may want to build up your policy overtime to monitor email traffic and gradually increase the security. This way you can spot false positives in the beginning and make sure your email settings are configured correctly.