Here is a good article explaining CAPE. CAPE is basically a way of telling how reasonably priced the whole market is. At the top of a bubble CAPE will be high, and in a slump, low.
If this was boiled down to one lesson it would mean to take caution when CAPE is high, be greedy when others are fearful ie when CAPE is low. This is because historically CAPE returns to its long run average.
UK Value Investor is a great site for those wishing to beat the market investing in large FTSE companies but with little effort. I think there might be a small subscription charge but Johns Kingham’s methods are pure common sense.