A "breakfast sandwich" is usually a food item containing eggs (pre-cooked, scrambled or fried), cheese, two slices of bread, and possibly sausage or bacon. The other ingredients are placed between the slices of bread, often this bread is some sort of roll but it may also be a pastry like a croissant. Miscellaneous foods placed between two slices of bread is known as a "sandwich", and the first meal of the day is known as "breakfast". So a "breakfast sandwich" is a sandwich containing foods typically eaten at the first meal of the day!

A "frozen" food is one that has been frozen, usually sold from the "frozen foods" aisle of a grocery store. Sometimes people will prepare meals in advance and then freeze them for storage. These items must be "defrosted" before eating, usually also heated to a desired temperature. This takes substantially less time than making the meal from scratch.

A "frozen breakfast sandwich" is a "breakfast sandwich" which has been pre-prepared, and then frozen. That saves the eater time & effort in the morning, since they only have to heat up the sandwich before eating, not go through the whole process of frying the egg, cooking the bacon, assembling it into a sandwich, & heating that. Even if the eater is also the preparer, it still saves time in the morning (at the expense of preparation time on a different day). For people who don't like getting up early or with long commutes, having pre-prepared frozen breakfast sandwiches can be a significant benefit.