The above statement is essentially court packing, in which extra seats are added into the Supreme Court. Essentially, it allows for more justices to be added to shift power between the two major parties.
Historically, the most (in)famous case of court packing is during Franklin D. Roosevelt's tenure, in which he asked for either a justice to retire before they hit the age of 71, and if they were to stay, to relieve their voting rights by appointing to them a assistant. On top of the assistant having the voting rights, the president was also allowed to add more justices for every judge that was over the " 'age-restriction' ".
In the case of your question, an addition of 6 judges would shift the balance of power heavily in favor of whichever party they align with. Take for example, today's Supreme Court make up. There are 9 Justices, in which 3 justices lean left (Sotomayor, Breyer, Kagan, leaning from a -3.9 rating to -1.5), 2 justices towards the center (Roberts and Kavanaugh, with a rating of 0.506 and 0.548 respectively), and 4 justices leaning right (Barrett, Gorsuch, Alito, and Thomas). Assuming the extreme, if, for example, all 6 justices added leaned left (Democrat), it would make the advantage shift to the left 6 points with a 9-6 split towards Democrat agenda favor. On the other hand, if all judges added leaned right (Republican), it would make the advantage shift to the right 6 points with a 3-12 split towards Republican agenda favor.
Of course, this is assuming that each justice would vote based on party lines, which is not the case most of the time.
Learn more about the Supreme Court and court packing, here:
https://brainly.com/question/1372733 - See demonwolf's answer.
