The Japan vs Poland game was happening at the same time as the Senegal vs Colombia game. At the end of the games Poland and Colombia were winning their games by one goal. Given the standings Colombia would go through as first seed with 6 points, while Japan and Senegal were tied with 4 points.
Japan and Senegal had the same number of points, goals, goal differential. But Japan held the tie breaker by having 2 less yellow cards than Senegal.
So instead of attacking in the stoppage time and push for a goal Japan took the safer way by keeping the ball. Had they pushed for a goal they would've been vulnerable to counter attacks, which could lead into goals for Poland or more yellow cards for Japan which would've reduce from their tie breaker over Senegal. Also Poland was already eliminated and were winning the game so they didn't need to push Japan to get the ball back. So Japan made sure to keep control of the ball without trying to score and hoped that on the other game Senegal wouldn't score a goal.
At the end the score of both games remained at 1-0. Japan made it to the round of 16 for having 2 less yellow cards based on fair play performance.