Short answer
Top coat is a nail product applied over color or a prepared nail to create the finishing role described by its label. That role may involve visible shine or another surface effect, but the category name does not guarantee a fixed drying time, wear duration, chip resistance or compatibility with every lacquer.
Read the role before application
Confirm that the bottle is a top coat rather than color, base coat or treatment. Read directions, warnings and storage information, and use it only as intended. Keep nail products away from heat or flame when labels require that precaution, work with appropriate ventilation and close containers promptly. Do not dilute a thickened product or mix formulas unless the manufacturer provides directions for doing so.
Apply color layers and top coat according to the products' own instructions. A wet-looking shine is not proof that underlying layers have settled, and surface touch is not a universal timing test. Use a controlled amount and avoid flooding the skin around the nail. If product contacts surrounding skin, follow the relevant cleanup directions rather than scraping aggressively.
Decide whether the finish is needed
Top coat is an optional finishing role, not a requirement that makes every manicure complete. A matte lacquer may be intended to remain matte. Reapplication should answer an actual label-supported maintenance need and the current appearance, not a rule that any visible edge can be sealed indefinitely.
Common questions
Does top coat always make nail color last longer? Do not assume a universal duration. Evaluate the specific product claim, directions, color formula and real use.
Must every nail look use top coat? No. Use it when its specified finish or maintenance role belongs in the chosen look.
Sources: FDA nail-care product information and FDA guidance for using cosmetics safely.