Let's look at the parts of the question:
Southwest Asia - this is today's Iran, Turkey, Syria and neighboring countries
North Africa- Egypt, Tunisia, etc
Let's already think, what do those two places have in common? Well, they are the craddle of the (western) civilisation, which can be traced back to ancient Egypt and Mesopotamia.
And this is why they are called a "culture heart" - this is the place where culture originated - for example, agriculture and writing spread from those places to other places.