Non-monogamy describes relationships that are not sexually exclusive but may not be explicit about the nature of the other sexual relationships. As it sounds, non-monogamy simply means that a relationship between two (or more) people is not at all times monogamous.
Some people will have a non-monogamous relationship where they tell each other about all their other sexual partners, others will have a don't ask, don't tell understanding.
Non-monogamy is not the same thing as cheating, as the people involved have openly discussed the rules and parameters of the relationship and agree on what they are, and what each individuals responsibility is.
Non-monogamy can be distinguished from polyamory in that it doesn't necessarily involve love, or relationship commitment. Being non-monogamous might just mean having casual sex now and then inside of a committed relationship.
As with all definitions, they change, and differ for people. There is no one definition of non-monogamy and people tend to find ways of describing themselves that fit their unique relationships best.