The above is the fourth verse of the Fourth Gospel ('John' - in the divinely-inspired 'King James' version of the Bible); and this is the first definition of what-Jesus-was.
Jesus had Life in him... and this Life in Jesus was the Light of Men...
When striving to understand the key terms of Life and Light we moderns are up against the fact that people used to think in a different way from ourselves. Through its history language has become less 'poetic'; more precise, narrow, materialist. To recover the meaning we need to recover the thought-world, the way of thinking - and we can understand these terms only in such a context.
The Life that was in Jesus, implicitly to a high degree, comes up repeatedly through the Fourth Gospel. This Life of Jesus seems to be like a solidification and concentration of creation. Verse 4 follows a statement that Jesus (the Word) was creator of this world ('All things were made by him...'). So, Life is (partly) creation.
I think we are being told that Jesus offers Man the possibility of an eternal participation in the work of creation. Through this Gospel, the Life that Jesus offers is contrasted with normal life, the mortal life that ends in death; whereas the creative Life is 'eternal', 'everlasting' - and to reach it fully we need to go-through death, be born-again.
The Light is, mostly, a name for Jesus himself (like the Word); but simultaneously contains other meanings of light such as brightness, goodness, and something we can follow in the dark (darkness being the opposite of Light, and the world regarded as generally dark)...
So we could say that the Light is an aspect of Jesus's nature, and also some of the qualities that Jesus embodies: qualities at the heart of his message and work: him having made us able to become 'sons of God', that is to become like Jesus himself, a creator of worlds.
And that Jesus is Light is also what guides us, and enables us to follow Jesus through death.