This operator matches the visibility of objects in the viewport with their render visibility, ensuring that what you see in the viewport matches exactly what will be rendered. It works with objects and collections.
It saves a lot of time in complex scenes that contain a lot of construction objects (like booleans, backup objects or different model stages) that suddenly appear in the final render but are hidden in the viewport.
What you see in the preview render is what you'll get in the final render.
Click on the button in the header of the Outliner. It works for collections too.
Sometimes you may want the opposite behavior of this operator, that is, to sync the render's visibility to the viewport instead.
Hold ALT and click the button, this will copy the render visibility into the viewport visibility of objects and collections.
Use the opposite behavior (ALT + Click the button) to quickly unhide objects in the viewport. Sometimes you can see the objects in the render but can't find them in the viewport, and since you can't select them, you can't locate them in the outliner even with the Sync to active toggle ON. So a quick fix is to use this operator in reverse.