What is the meaning of ?

Short for 槓子 (kantsu): a set of four identical tiles

a call for such a set (compare English kong):

  1. for a 明槓 (minkan, open kan), the player calling should have a 暗刻 (ankō, closed triplet) in hand and the current discarded tile is the fourth tile
  2. for a 加槓 (kakan, added kan), the player should have an 明刻 (minkō, open triplet) beforehand and the fourth tile must be self-drawn
  3. for an 暗槓 (ankan, closed kan), the player calling must simply have all four tiles in hand

for a 明槓 (minkan, open kan), the player calling should have a 暗刻 (ankō, closed triplet) in hand and the current discarded tile is the fourth tile

for a 加槓 (kakan, added kan), the player should have an 明刻 (minkō, open triplet) beforehand and the fourth tile must be self-drawn

for an 暗槓 (ankan, closed kan), the player calling must simply have all four tiles in hand

Source: wiktionary.org