At will is an employment policy that says within certain limits (set by Federal or state laws) you can be fired for any reason or no reason at all. Similarly, you have the right to walk off the job/quit for any reason or no reason at all. Among some of the limitations are the usual you cannot be fired for race, ethnicity , religion, etc.

Many people confuse this with "right to work" which is a different policy altogether.

it means if the company gets a wild hair up their asses they can fire you for just being you, no reason needed.

Steve nailed the answer!

You can quit for any reason and they can fire you for any reason.