What does "significant" mean here
Meaning that there is significance to whitespace, as opposed to C where you could cram it all onto one line if you wanted. To put it another way, C doesn't care how you indent, if you even do it at all. Python cares, and will not run your program if you get the whitespace wrong.
Whereas I think you read it as "there's lots of whitespace in Python", which is not what was intended.