API Region SetAlpha

defines the opacity of a, normally relative to its immediate (if any) unless overriden by.

region:SetAlpha(alpha)

Arguments

 * alpha : Number - A proportion where 0 means fully transparent (invisible) and 1 means fully opaque or as opaque as its parent.

Example
Makes frame  translucent, more visible than not: MyFrame:SetAlpha(.75);

Fade the whole UI so that it becomes barely visible: UIParent:SetAlpha(0.1);