Short version that doesn't require technical knowledge is Red Bull Racing (RBR) were using a system on their brakes that wasn't allowed but it was suspected that teams were using anyway. The rules were publicly clarified before the Miami GP. Ever since then Max Verstappen and Red Bull in general have started to lose pace compared to other teams, breaking Max's long run of victories. The tweet is suggesting that Max's break issues that caused him to DNF in the Australian GP were due to a fault in this system and the loss of pace is due to it's removal.
Edit to correct detail that I mis-remembered. Max did not DNF in Miami, in fact he finished 2nd. It was Australia that he retired from.
"Many that live deserve death. And some that die deserve life. Can you give it to them? Then do not be too eager to deal out death in judgement. For even the very wise cannot see all ends."
-Gandalf the Grey / J R R Tolkein