Lesson 1: Zwischenzug

To the right white captures the knight on f4 and black captures the bishop on f1 and white has to play Bg3 before re-capturing blacks bishop because then whites bishop would be left hanging.

A Zwischenzug is a move you play in-between a move and your expected move, for example in this position after black captures the knight on f4 and white captures on f1 evening out the material white is expected to re-capture but instead attack the queen by moving the un protected bishop to a protected square and after the queen retreats white finally recaptures and goes on to win the game.