From the box cover:
"Bolero - a tactical game for two.
A rousing, ingeniously simple game, where winning demands all your acuity and maneuvering skills."
Players take turns to place their markers on the board. Both players have 64 markers numbered 1-4, 16 of each. The board is a 4x4 grid of 2x2 squares numbered 1-4. The played marker always goes to a square with a same number.
The player who first creates
1. a row of 4 same numbers horizontally, vertically or diagonally,
2. a row of 4 different numbers horizontally, vertically or diagonally or
3. a 2x2 square of 4 different numbers
with his own markers wins the game.