The Loop toolbar's second icon from the left is a small clock with a knife and fork on the sides. This is the pre-meal tool. The tool will be colored grey until you define a set of blood glucose targets for it in Loop settings (within the Correction Range section). Once you set pre-meal targets in that section, the icon will be colored green and available for use.
The pre-meal override target can be used to as an easy way to get a small amount of insulin delivered before a meal in order to help control post-meal blood glucose spikes. It's not designed to replace a traditional pre-bolus, but rather as a more gentle way to build up some pre-meal insulin activity.
If your normal target is 100-110 mg/dL and pre-meal target is 80-80 mg/dL, for example, Loop will give you an extra push to get you to the lower target number before the meal. This early insulin brings you into the meal with a mini-prebolus. The pre-meal target, when activated by pressing on the icon, will stay active for one hour, until carbs are entered, or until it is manually cancelled...whichever comes first. Setting an override will also cancel pre-meal target.
Loop will adjust any insulin bolus as needed based on the extra insulin provided during this pre-meal time.