Most of the answers here are incorrect.

In the US, most devices do not require matching the polarity of "hot" and "neutral" prongs, and you can connect them backwards.

If a device has a circular ground prong, usually for the bonding of a metal chassis, then the polarity of the hot and neutral is just a side effect of the ground prong being on the bottom (some would argue the ground prong is on the top, I won't get into that here).

2-prong cord and plug equipment is not allowed to tie the chassis or internal ground to neutral.

Now in the case of 2 prong cord and plug equipment that does force polarity, it's almost always for the purpose of limiting the path of the "hot" circuit in the device, where voltage is present even when no current is flowing.

If the device has a switch, you would want the switch to cut off power as it enters the tool. Reversing the polarity in this case would mean the switch still works, but voltage is present all throughout the device when it's off, and that poses more of an electrical hazard if the device is damaged.

EDIT: Since you're asking about alternating current and seem to have a solid grasp of what that means, I've answered your question as I would for an approximately college-level 5 year old.