This is a wildcard; it matches each element. Thus, in your example, this means that each element that is a descendant of the elements has a class of "center-wrapper".
According to W3C official docs, it is called a universal selector, see http://www.w3.org/TR/css3-selectors/#universal-selector :
The universal selector, written as the Name assigned by CSS (CSS3NAMESPACE) with an asterisk (* U + 002A) as the local name, is the name of any element type.
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