Help with question from Sipser chapter 1
I think I finally understood the concept.

This:

$A =\{0^ku0^k  \ | \ k \ge 1 \ and \ u \in \sum^*$}.

Means that A has strings that starts and ends with the same number of zeros.

For example, the string:

$0000000100$

Is recognized by A because:

$00|000001|00$

In the middle we can put any number of zeros and shrink the zeros at the edges.

$0^k10^k$ is another language that contains strings with equal number of zeros at the beginning and end separated by a 1. In this case, string from this language are accepted by the the first one but that doesn't means that the second language could be regular.