"aiming mode" refers to a state or setting where you are focused and directing your efforts towards a specific goal or target. It means that you are taking aim and concentrating on achieving something. Full definition
You tug the left trigger to go into aiming mode while firing with the right. (ztgd.com)
Some very important upgrades are hidden behind these, including a precision aiming mode that requires you to hit a firing range to achieve a specific score. (attackofthefanboy.com)
Or set the Move button to go into aiming mode without having to hold it down. (blog.us.playstation.com)