The leaves of Monstera leave curl in order to reduce the amount of exposed surface area and minimize water loss through transpiration. This usually happens because of some issue that is causing water loss, like heat stress, underwatering, damaged roots, or insect infestation.
It could be a sign that something is wrong with the plant. The most common reason for curling leaves is insufficient humidity, so if you live in a dry climate, make sure to mist your plant regularly. Curling can also be caused by too much direct sunlight or drafts from doors or windows. If you think one of these might be the problem, try moving your plant to a different location. If the leaves are still curling, it might be time to repot into a pot with drainage holes to improve drainage.