This rhetorical device is used to same something positive with the use of a word or phrase usually reserved for a negative context. For example, one could say “I’m not unfamiliar with it“. Or to translate, I know a lot about the thing that you are talking about. “You are not average player, aren’t you?” which means you are nothing but average. Or one could say “This is not my first rodeo” which means, trust me I have a lot more experience than you give me credit for.