What was FDR's risky scheme?

President Franklin D. Roosevelt was very unhappy that the Supreme Court had declared some of his New Deal programs unconstitutional, such as the National Industrial Recovery Act and the Agricultural Adjustment Act. There was some thought that the Wagner Act and the Social Security Act might also meet the same fate. President Roosevelt felt that some of the judges on the Supreme Court were out of touch with reality and were too old to be serving on the Supreme Court.
President Roosevelt developed a plan that many people viewed as risky. It was known as the court-packing plan, which would allow President Roosevelt to appoint one judge to the Supreme Court for each current justice that was at least 70 years old and that didn't retire. Many people felt that President Roosevelt was violating the principle of separation of powers by proposing this plan. They believed that this plan gave the president too much power and too much influence over the Supreme Court. Some people believed this proposal was the first serious mistake of his presidency. The plan never went into effect, partially because the Supreme Court didn’t declare the Wagner Act and the Social Security unconstitutional, and also because Congress voted against its implementation.
https://www.history.com/this-day-in-history/roosevelt-announces-court-packing-plan