Don't Ask Don't Tell (DADT) Polyamory is a relationship structure in which a partnered person is permitted to have additional relationships on the condition that their partner does not know meet any other the other people involved or know anything about those additional relationships. These additional relationships may be sexual, romantic, queerplatonic, etc. This is not the same as cheating because all people involved are aware of and consent to the possibly of other relationships- but they choose not to learn any details of these other relationships.
One or both of the people in a DADT relationship may have additional partners. If only one partner has additional relationships that is because the other partner chooses to not to have additional relationships.