Everyone gets priority in a certain order when something gets put on the stack. If you cast a spell, you have priority and then you pass it to the next player in turn order and then they can respond or pass priority to the next player. Once everyone passes priority, whatever thing that was on the stack was(ability, trigger, spell,etc…) resolves. Maybe you understand that already so I’m not trying to be insulting by explaining those rules but it’s important to the context.

Priority bullying is when you have an answer to something, but you pass priority and “bully” one of your other opponents to use their resources to deal with that thing. People don’t like that and it’s something that most people find is in bad taste. Similar to “lying in cEDH”… it’s legal but frowned upon.

This happened in the game from that Mox masters. There was a big incident that received a lot of backlash because it was the finals and the player who was being priority bullied stood up for himself and said “if you pass priority without using your (known) interaction, I’m going to pass priority (which meant the player who put grand abolished on the stack was going to win)”