Time will tell means that we will know the result of something after some time passes. I suppose this expression has a little skepticism attached to it, in the sense that the person who says it may doubt the results of the thing in question. Here's an example:You: I think the new program to reduce costs will be really successful. What do you think?
Your coworker: Well, it sounds good, but I'm not sure it will work... Time will tell.
