The phrase "not to mention" is an idiomatic expression that is used to introduce an additional point, often something that is related to the topic being discussed, but that may not be the main focus.

It is used to add emphasis or to draw attention to something that may not be immediately obvious, but that is worth mentioning nonetheless.

For example:

The new car has great gas mileage, not to mention its stylish design.

The city is known for its historic landmarks, not to mention its lively nightlife.