Fianchetto
A fianchetto is developing a bishop to b2 or g2 (or b7 or g7) after moving the knight’s pawn one square.
The bishop sits on the long diagonal and watches the center from the flank. The pawn on g3 (or b3) is both a home for the bishop and a hook the opponent can later attack with h- or a-pawns.
Bg2 is the kingside fianchetto. The bishop looks toward a8 and helps castle. The same pattern on the other wing is b3 and Bb2.
Once you fianchetto, the bishop is often your best piece. Trading it without a good reason leaves holes on the squares it used to cover, especially around a castled king.