I in You, You in Me

In John 15:4-5, we read, "4 Abide in Me, and I in you. As the branch cannot bear fruit of itself, unless it abides in the vine, neither can you, unless you abide in Me. 5 I am the vine, you are the branches. He who abides in Me, and I in him, bears much fruit; for without Me you can do nothing." (NKJV) Just what does, "He who abides in Me, and I in him," mean?