Devices drawing current
Your phone has in internal battery charger which controls how it charges its battery.

Based on what you have said it is capable of using (or drawing) enough power from the charger that the original charger was rated at 2A (2 amperes).

If you use a charger capable of (in this case) only 1 ampere, then it is possible that the phone may "draw" more than the 1A the charger is rated at, which may cause the charger to overheat (as it 'attempts' to provide more than it is designed for).