What does it mean that the fall of the Roman empire and the start of the European renaissance bookend the period of the Middle Ages?

When two things "bookend" something it means that they mark the start and end of that thing (like the covers of a book).

The fall of the Roman Empire is considered the start of the Middle Ages as it was the fall of the largest civilization in Western Europe. During the Middle Ages a lot of technology and studies of arts were lost. These things were not "rediscovered" until the European Renaissance (in Western Europe, much of the rest of the world was still developing perfectly fine). The Renaissance is considered the end of the Middle Ages. The fall of the Roman Empire and the start of the Renaissance bookend the Middle Ages as they mark the start and end of that time period respectively.