A lunch hook is a term for a light anchor that can be quickly deployed and retrieved, and is used for short stops. Lunch hooks are too small to be used during rough weather or if you are leaving your boat for long periods (such as overnight). Technically, a lunch hook should be used only while you are on board the boat; otherwise you would use a slightly larger anchor which might be called a day anchor or a fair weather anchor. However, it is common to refer to any light, temporary anchor as a lunch hook.
While the exact requirements of a lunch hook will depend on your boat, when and where you use it, the type of anchor, and the length of your anchor chain, the general rule of thumb is half a pound of anchor per foot length of boat. (This is in comparison to a pound per foot for regular anchors). Sometimes much smaller anchors, about 0.25 pounds per foot, are recommended. Other than size/weight, lunch hooks are no different from standard anchors.
A well-stocked boat will probably also have a working anchor and a storm anchor.