If our Sun is, as we believe, a perfectly normal 4.5 billion-year-old main sequence star, we would expect that, say 3 billion years ago it would be largely the same as another main sequence star of that age. And, 3 billion years ago, we would thus expect that the sun would output around 70% of the energy it does today. Unfortunately, this is too little to sustain liquid water on the surface of the Earth. And yet, we know that there was.
This, then, is the faint young Sun paradox.