Staggering is preventing an opponent from effectively attacking or fleeing. This can be done by stunning, booping, blocking, or trapping. Using stagger effectively can help you utilize a fancy time-wasting tool used at high-level play. It is a great maneuver because it is not difficult, but it does require the slightest of coordination.
Take for account this scenario: your team acquires five quick kills, but an Ana got away. Your whole team goes and overwhelm(s) her and destroy(s) her. What has your team accomplished? Absolutely nothing. There was no way that the Ana would win a six vs one. The worst she could do is build up some ult charge.
Instead, think of it this way: Ana gets away, but Lucio boops her into a one-way room. She tries to get out, but she is blocked by a Mei wall. The wall goes down, Ana tries again, but she is met with a flashbang. Then she is finally killed. Analyze what has been done here.
Ana, who may or may not have her ult, will not respawn for a good amount of time. This takes a huge toll on the enemy push. Not only can they not push, but they are also left with time ticking down. Your team could even engage and repeat, causing them to waste all their time without even pushing if they do not regroup at spawn. The effect the respawn timer and the walk back from spawn have can be used for your advantage and help win games.
Example: https://youtu.be/Go6wmBPQft8?t=251 "Stagger her. Don't kill this Ana yet."
I hope this quick one helps. It is a bit team oriented but very simple.
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