The way I understand the concept of “idle talk” (which could very well be incorrect) is meaningless chatter, talking just to hear oneself speak, or talking just for the sake of the distraction of talking. This of course can include outright negative forms of communication such as gossip and slander, but what is being said doesn’t necessarily need to be negative. When we speak we are asserting our own will. When we speak needlessly or for vain reasons then we are engaging in a prideful activity. This is why idle talk is sinful.
However, I would not say that one is restricted to only talk about “sacred things”. Clearly (as another poster pointed out) we need to communicate in order to survive. And I don’t think we’re restricted to purely functional communication either (i.e. requests, commands, etc.) The Lord used very beautiful and colorful language to communicate with others, especially in His parables. And the Bible is filled with beautiful poetic language (i.e. the psalms, proverbs, etc.) God of course did create speech, so speech is good so long as it isn’t disordered.
Anyway, I feel I’ve probably taken way too long to explain something quite poorly. I hope I was able to help you a little.