MTF, modulation transfer function, is how much contrast the lens still has at a given spatial frequency. In human: can this glass still tell a black bar from a white bar at the pitch of your pixel? A lens page that says "8 MP" is trying to answer that with a sticker. The sticker is not a measurement.
In one sentence. If the lens is mush at the Nyquist of the sensor, you did not buy megapixels. You bought a smoother blur.
Nyquist in one paragraph
A 2.0 µm pixel has a Nyquist frequency around 250 lp/mm. The lens needs contrast there, not only in the centre at F/2.8 on-axis. Corners, full field, the F-number you will actually use: that is the test. A wide M12 that is sharp in the middle and gone at 70% field is a common purchase and a common regret.
The newspaper-across-the-room test
You can count the columns on a newspaper at two metres, or you can see a grey slab. Your eye has MTF. So does the glass. Buying a finer newspaper (more megapixels) does not help if you are looking through a steamy window (the lens).
How to read Camemaker
The lens library has a resolution field. Treat it as a class, then look at image circle, F-number, and the sensor pixel size together. If you are pairing 1.4 µm pixels with a general-purpose M12, ask for an MTF plot or measure one. We do not invent a lp/mm number that the page does not publish.