Fianchetto (Italian for "little flank") means developing a bishop onto the long diagonal by pushing the knight's pawn one square and sliding the bishop into the gap: g3 then Bg2, or b3 then Bb2. From that perch the bishop watches the longest diagonal on the board, straight through the center to the far corner.
A fianchettoed bishop also becomes a bodyguard: when you castle behind it, it shields the king's weakest squares. That's the double deal the setup offers, a great diagonal AND a safer king. The price to know about: if that bishop later gets traded off, the holes it guarded stay weak for the rest of the game.
8
7
6
5
4
3
2
a1
b
c
d
e
f
g
h
White has played Nf3 and g3. The g2 square is ready for its new tenant.
Complete the fianchetto.
8
7
6
5
4
3
2
a1
b
c
d
e
f
g
h