Sentence:  A star, or other point-like source, viewed through the atmosphere can be seen by eye to fluctuate in intensity. An extended object has a brightness distribution that is convolved with the seeing disk. That is, the object's brightness distribution can be thought of as a series of point sources of different brightness, each one 'blurred' to the size of the seeing disk and each seeing disk is <WSD>scintillating</WSD>. If the eye could spatially resolve each of these points, it would see brightness fluctuations across a source. However, the resolution of the human eye (≈ 1′) is much poorer than the seeing (≈ 1″). ... The result is that the eye perceives an extended source as steadily shining. 
Meaning of the word marked with <WSD> tags:  Of a star or other celestial body: to vary rapidly in brightness; to twinkle.