Toe spring is when the front of the shoe, where the toes sit, curves upward. If this is rigid, then it perpetually forces the toes upward and can lead to an imbalance in the foot (overstretched plantar fascia, shortened extensors). Over time this can lead to foot pain and dysfunction. That's why barefoot shoes have no rigid toe spring and are completely flat. Note that the problem is when the toe spring is rigid. Many barefoot shoes get "acquired toe spring" after they've been worn. As long as the shoe flattens out when you stand in it, it shouldn't be a problem.