The apostle simply means that Jesus was historical; he really was a physical being, with flesh and bones. John and many others were eyewitnesses of him who has life in himself from eternity, but has entered time and space in the flesh. This may serve as a counterargument to the Docetic view, which held that Christ did not assume humanity, but rather came as a spirit or a ghost. The wonderful reality of Christ’s incarnation and coming into the world is that his fellowship with the Father also entered time and space. This means that John, the disciples, and all those who saw and touched Jesus, heard the very words of God and touched God incarnate.1
1 That which was from the beginning, which we have heard, which we have seen with our eyes, which we looked upon and have touched with our hands, concerning the word of life