Outposts

An outpost is a square for a knight or bishop that's defended by one of your own pawns, and that no enemy pawn can ever attack -- not now, and not later either, no matter how the game continues. That second part is what makes it different from an ordinary safe square: a piece can sit somewhere quiet today and still get chased away by a pawn push next week. An outpost is permanent.

To check whether a square is an outpost, look at the files immediately to its left and right. If an enemy pawn already stands on one of those files, could it ever march up far enough to attack the square? If both enemy pawns on those adjacent files are missing, or have already passed the square (so advancing them further can't reach back), the square is unassailable by a pawn forever.

Knights benefit the most from outposts because a knight is a short-range piece that badly wants a stable base to work from -- once it's parked on an outpost, only a piece (never a pawn) can ever challenge it, and trading a bishop or knight to dislodge it is often a real concession for the opponent. Bishops can occupy outposts too, though it comes up less often since bishops are already long-range and less dependent on a single anchor square.

Outposts typically appear on the 4th, 5th, or 6th rank (from the piece's own side), right in the hole left behind when the opponent's pawns advance past a file. Spotting them is really pawn-structure reading in disguise: an outpost is the flip side of a weak square in the opponent's camp.