What does NSA mean by 'Analyzable'?
Analyzable in this case means "simple to study".

If your cipher consists of a small function that mixes a few things together, and then you repeat that often then your cipher is more easily understood and more easily fit into previous research knowledge than a overclomplicated large design. This is the case with Simon and Speck.

Being easy to analyze is a good property for a strong cipher. If people fully understand it and fully checked everything and still couldn't find a weakness then it adds confidence in the cipher. A more complicated design is more likely to have a "hidden weakness" - because more things can go wrong.