Weapon Sync means that your main-hand and off-hand weapons are swinging simultaneously. This is important because if the swings are landing within 0.5 seconds of each other, only one Flurry charge will be consumed for two melee hits, drastically increasing the 30% Haste buff uptime. You can know whether you are synced properly or not with this WeakAura, and force the sync with this WeakAura or by taking an action which resets your swing timer, such as using Feral Spirit or using a Noggenfogger Elixir.