Wild men who caught and sang the sun in flight, And learn, too late, they grieved it on its way, Do not go gentle into that good night.
The speaker describes another kind of men – those who don't allow themselves to fade quietly away into death, "Wild men" (line 10).
What sort of men are we talking about? The kind who captured the world around them in their imagination and celebrated it – "who caught and sang the sun in flight" (line 11) – only to discover that the world they celebrated was slowly dissolving around them as comrades age and die.
Here the sun represents the beauty that exists in the mortal world, and its "flight" across the sky represents the lifespan of people living in this world.
"Flight" also suggests that it moves rapidly – our lives are just the blink of an eye.
So just when you think you're partying to celebrate birth and life, symbolized by the sunrise, you find out that you're actually mourning death, symbolized by the sunset.