Nearly every time I’ve heard this quoted it is applied to heaven, and we are told that heaven contains for the Christian things none of us have seen, heard, or can imagine. But is that what Paul is saying? Not at all.
The context of the passage is contrasting the wisdom of this world and its leaders with the wisdom of God. Paul argues that the unconverted man does not understand the plan of God in the redemption that is in Christ, and he quotes Isaiah to say that what God has done for those that love Him is not seen, heard, or understood by the heart of natural, unconverted men. But in the very next verse Paul says, God has revealed His purpose unto His people by the Spirit of God.
In summary, the verse teaches that all God has done for his people is not understood by the unconverted, but it is understood by those born again of the Spirit.