Acting idly means behaving in a lazy, slow, or aimless way. Your idea of a perfect vacation might be lying idly on the beach all day.
If you're idly chatting with your friend, there's a casual, nonchalant tone to your conversation, and if you lounge idly on the couch on Saturday morning, you're lazing around with no real purpose. The adverb idly describes any action that isn't particularly active, and it comes from idle, "lazy or unemployed," and its Germanic root meaning "worthless."