Some varieties of tomato develop a physiological leaf roll. 'Beefsteak', 'Big Boy', and 'Floramerica' are three susceptible varieties. The margins of leaves roll inward toward the central vein, and leaflets become leathery.
Fluctuating soil moisture, excessive heat, or heavy pruning are the usual causes. The problem is mostly cosmetic; tomato yields are unaffected. If you'd like to avoid the problem, grow a variety that is less susceptible to leaf roll, maintain consistent soil moisture by mulching and watering as necessary, and avoid heavy pruning of your tomatoes.