i. An image can be considered as a two dimensional array say f(x,y). Now every image that is seen on the monitor is actually a matrix or a two dimensional array.

ii. Each element of the array is called a pixel. Whenever we see an image on the computer screen, it is actually a matrix which consists of pixels and each pixel is considered to be a sample.

iii. Thus more the number of pixels, means more samples which results in higher sampling rate and hence better spatial resolution.

iv. Now coming to grey level, we know that each pixel will carry a value. This value of each pixel is known as grey level. Let us consider we have only two bits to represent the grey level, in this case we would have $2^2$ different grey levels.

v. Suppose we have 8 bits to represent the grey levels, now in this case we will have $2^8$ i.e. 256 grey levels. More number of grey levels will lead to better tonal clarity which will in turn help in having a better resolution.

Thus quality of picture depends upon the number of pixels and grey level that represent the picture.