Place one of the digits 1, 2, or 3 into each cell, one digit per cell, so that each digit appears the same amount in each row and column. Some digits may already be placed for you. After placing all digits, each white cell must touch at least one cell with the same number along an edge, and each gray (outlined) cell must not touch any cells with the same number along an edge.
(Rules from PGP IB, example from WPC 2019 IB. Rules have been generalized to arbitrary grid size)
History of the puzzle
Hans Eendebak (the Netherlands) invented the puzzle in 2012, according to Richard Stolk.