what are frequencies in a digital image? I don't completely understand the term " frequency" with respect to digital image processing.
for 1D electrical signals , its pretty easy to understand , visually too.
higher the frequency - more packed( or closer) the signal looks in a given time interval than its lower frequency counterpart.

But when it comes to images , with parameters like intensity, 8-bit gray scale, pixel value,etc. - what does frequency mean ?
In images, intensity means amplitude and you can see it as in 1D case.
More packed (or change of intensity) in 2D area is higher frequency.
Note that, this way is just visual approximation by human's eyes.