A more thorough explanation:

At-will employment is a type of job where the employee can quit at any time, and the employer can fire the employee for any reason, as long as it's not illegal. This means that the employer doesn't need a good reason to fire the employee, and the employee doesn't need a good reason to quit.

For example, an employer can fire an at-will employee if they're not doing a good job, if they need to cut costs, or if they just don't like the employee. However, they can't fire an employee because of their race, religion, sex, or because the employee reported harassment or exercised a legal right.

At-will employment is the default in most states in the US, except for Montana. Employers can require employees to sign a contract that says they're an at-will employee.

For instance, if an employee is not meeting their job requirements, the employer can fire them without any legal consequences. However, if the employer fires an employee because of their race, that would be illegal and the employee could take legal action against the employer.