Ice hunter - algorithm explanation

Hi Zax,
Sorry but I don’t have a problem with the code itself, rather the level (ice hunter/ northwest) comments and hints explaining the algorithm, explaining what is necessary, and more importantly why.

So the question is rather trying to understand the algorithm referred to by these levels: substring search by shifted index.
The Hints mention it’s a well known algorithm but it does not explain it, it only explains nested loop.
I am looking for more guided explanation of the algorithm itself.

The coding part of it has already been covered in past forum/Q&A discussion, fore example:

sorry I cannot paste all the links, new users cannot paste more than 2 links.

But I am not after coding help, rather algorithm explanation.

Thank you,