universe was first opaque and then became transperent , how?

in book "dark matter , dark energy - the dark side of universe" , it says universe was opaque when it was young enough and only about 400,00 years after the bigbang , it became transparent.

my first question is , what does it actually mean by "transparent" and "opaque" (i do know the meanings of both the words" but i cant really comprehend their true meaning in the sense od universe. my second question is , why was earth first opaque? and my third question is , how come it became transparent from opaque .