Domain reputation means emails from your sending domain (SPF and DKIM) are less likely to get filtered to a recipient’s SPAM folder or inbox.
The definitions of SPAM below includes mail detected as SPAM by Gmail’s Spam filter, and mail reported by users as SPAM.
BAD: A history of sending an enormously high volume of SPAM. Mail coming from this entity will almost always be rejected at SMTP or marked as SPAM.
LOW: Known to send a considerable volume of SPAM regularly, and mail from this sender will likely be marked as SPAM.
MEDIUM/FAIR: Known to send good mail, but has occasionally sent a low volume of SPAM. Most of the email from this entity will have a fair deliverability rate, except when there’s a notable increase in SPAM levels.
HIGH: Has a good track record of a very low SPAM rate, and complies with Email sender guidelines. Mail will rarely be marked by the spam filter.
Please keep in mind that SPAM filtering is based on thousands of signals and domain reputation is just one of them.
Here is a sample Domain reputation from Google Postmaster: