Two pixels are adjacent if they are neighbors and their intensity level satisfies some specific criteria of similarity.
Example (for binary images):
In binary images, 2 pixels are adjacent if they are neighbors & have some intensity value either 0 or 1. Here it doesn’t mean if a pixel is 0 its neighbor should have a binary value of 0 and vice versa for 1 to 1.

Suppose the above HTML structure is an image. and In the above image when we consider p(4,6) which is almost in middle and black. so this pixels adjacent is :
1. 3,5
2. 3,6
3. 3,7
4. 4,5
5. 4,7
6. 5,5
7. 5,6
8. 5,7
which is in 8 neighboring pixels. All marked in red