He is talking about making your sensitivity when you are aiming down sites the same as your hip-fire sensitivity. When you are scoping in, your FOV changes, which causes your sensitivity to be off. In order to make it the same, you need to change the aim sensitivity while zoomed. This YouTube, and many others, recommend changing the setting so your aim is 1:1, aka the same when you are scoped and un-scoped. Different hero's scopes have different zooms, so there are two values you need to change:
Relative Aim Sensitivity While Zoomed 51.47% (Ashe)
Relative Aim Sensitivity While Zoomed 37.89% (Widowmaker/Ana)
Remember, you need to select a specific character in order for this setting to appear in the control settings.
DaddyJuanito's post in Overwatch 2, FOV of each character in correlation to mouse sense was marked as the answer