“Employment at will” is a legal principle that essentially means both the employer and employee are free to enter and leave the employment relationship at anytime. There is no contract of employment for any specified period of time. The only obligation, absent some specific agreement, is for the employer to pay an employee for the hours or days worked according to the agreed upon wage at the time. What it means for an employee in particular is that the employer can hire, fire, discipline, increase or decrease a wage, and change any other term or condition of employment for a good reason, a bad reason or even for no reason at all. The only limitations are that the employer may not take such actions doing so violates some written law like the federal or state employment discrimination laws, wage laws, medical leave laws, and the like.
Whether the “employment at will” applies depends on state law. Wisconsin, like majority of states, is an employment at will state.