It's when a machine teaches itself. No programming. All you do is tell it the rules of the game and whether it has won or lost. A training set may or may not be supplied.Then what is it [machine learning]?
When AlphaGo defeated Lee Sidol to become Go champion of the world I knew that was the biggest engineering breakthrough of my lifetime. It was much more impressive than chess because the game space of Go is far greater than the number of particles in the visible universe. Go cannot be mastered by brute force.
AlphaGo was given a training set of Go games played by experts. Shortly afterward AlphaGo was defeated by AlphaZero, which was given no training set whatsoever. Playing against itself, AlphaZero became world chess champion after nine hours of self play, defeating Stockfish 8. The latter is a traditional AI that searches about 27 million positions per move. AlphaZero searched about eighty thousand.
It took AlphaZero 34 hours to become world Go champion entirely via self play. As you can see, it makes little difference what sort of game the learning algorithm is applied to. It can play Donkey Kong, Breakout, and so forth, these being much easier than Go. Instead of alternating turns players make their moves in real time, but this doesn't matter.
The next step was AlphaStar soundly defeating two of the the very top players in the war game of Star Craft II. This game is largely about strategic planning/logistics in a situation in which most of your opponents moves are unknown. AlphaStar achieved its mastery in fourteen days of self play after absorbing a training set of human games. Some said the computer had a speed advantage but the computer made its moves at about half the rate of a top human player. https://www.deepmind.com/blog/alphastar-mastering-the-real-time-strategy-game-starcraft-ii.
Surely the armed forces are hard at work applying this technology to real world battles. For all we know they may already be at use in the field.
Only two years separated the triumphs of AlphaZero and AlphaStar. I would have thought much more would be necessary. The revolution was going much faster than I expected. The results are apparent in the autonomous soldier robots produced by Boston Dynamics. Simulations have been developed accurate enough that the machine learning can take place in the simulations. Ten years ago humanoid robots were doing the Alzheimer's shuffle. Now they can perform standing backflips.
Such revolutions are unstoppable. The cat is out of the bag. All you can do is hope that the positive results outnumber the negative.