Meaning: The baby pigs were very happy in their pen.
Choice of Words: "Piglets" are baby pigs. "Squealed" means to make a high-pitched cry. "Happily" means feeling or showing pleasure or contentment. A "sty" is a pen or enclosure where pigs are kept. So, "Piglets squealed happily in the sty" means that the baby pigs were making happy noises while they were in their pen.