Boundaries are closed curves or a collection of closed curves which prevent the entry or exit of a tool from the bound area. They are used to limit the area of a model that you want to machine to prevent any unnecessary machining, which helps to increase the efficiency of your toolapths.
They can also be used to overlap different machining strategies to areas of a model, which is useful if you require a particular toolpath strategy for some given geometry.
Any toolpath that uses a boundary should, typically, have the same Tolerance and Thickness settings as the boundary.
You can access many of the functions you need to create and modify boundaries from the Boundary tab on the ribbon.