It means that God knows your desires, intentions, and motivations. He knows what's in your heart even before you do. This can be both a good and a bad thing. It's good because it means that God is always there for us, even when we don't know how to help ourselves. But it can also be bad because it means that He knows all of our secrets – the things we're ashamed of and would never want anyone else to know. No matter what, though, we can always trust that God knows our hearts and loves us unconditionally.