Innistrad: Crimson Vow introduced a new keyword geared towards dealing with all those pesky werewolves and vampires. Training happens as follows: whenever a creature attacks alongside a creature with greater power, you may place a +1/+1 counter on the creature with the training keyword. Think of training as a reverse version of the Guilds of Ravnica's mentor keyword.
It's a pretty straightforward keyword, so whenever a creature with training attacks and there is another attacker with greater power, the creature with training gets a +1/+1 counter. In terms of flavor, training features on human creatures in Innistrad: Crimson Vow. This is meant to represent how humans have to tackle the ongoing plight by keeping sharp and growing through combat. With this, you'll only see training on the green and white human creature cards in Innistrad: Crimson Vow.
Here are some notes and tips to keep in mind when using training:
Sounds like fun? Want to add to your deck? Check out all the cards that feature training!