Each of the millions of photoreceptors in the retina measures the number of light particles - photons - it has absorbed.  This is a ‘sample’, or estimate, of the amount of light coming from a particular location in a scene.  The more of these samples we have, the more detail can be captured.  This number of samples is referred to as ‘resolution’: the more samples, the higher the resolution.  This is equivalent to the measurement on digital cameras: a 5 megapixel resolution camera is making 5 million measurements (samples) of the scene you are taking a picture of. 

On the right here is a representation of what a particular scene might look like if you had the number of photoreceptors listed under the picture:

(just a sidenote going back to our discussion of the anatomy of the retina, don’t forget about all the retinal structures (‘collector cells’, blood supply) that sit *on top* of the photoreceptors, blocking some of the light getting to them: