Security camera resolution is the number of pixels in a recorded image, such as 1920 by 1080 or 2560 by 1440. It sets how many detail points a camera captures, but usefulness depends on how much of the frame a subject fills and on the light available.
Megapixels describe a sensor, not a result. A wide 4 megapixel camera covering a whole lot can put fewer pixels on a face than a narrow 1080p camera aimed at one door.
Resolution and pixel density
Measure detail as pixels across the target, not pixels per frame. Halving the field of view doubles the pixels on a subject, which is often cheaper than doubling the sensor.
The limits of more pixels
Small sensors crowded with pixels gather less light each, so low-light images turn noisy. Bandwidth, storage and recorder load also rise with pixel count, and every one of them has to keep up.