In John:15:4-5[4]Abide in me, and I in you. As the branch cannot bear fruit of itself, except it abide in the vine; no more can ye, except ye abide in me.[5]I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing., 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?