Toe-in is simply the front wheels' parallelism. The front rim of the wheels ought to be about 1/8 inch closer together than the rear edge of the wheel. That's so that when the rolling resistance of the tires is applied to the steering linkage the two front wheel are pushed to be almost exactly parallel to each other.
Your car's front wheels are not parallel to each other, therefore the tires are being worn by steering against each other. Toe-in is a simple adjustment of the tie rods' length that most alignment shops will do without a 4 wheel alignment etc.
Hitting a curb hard or turning the wheel against a parallel curb can distort the toe-in.