A person does something willfully if they made a conscious decision to perform, or not perform, a certain act. Whether a person does something willfully can be very important in a legal setting because it can determine whether a person is guilty of a crime or how severe their punishment should be.
Whether a person does something willfully is especially important when it comes to criminal cases and criminal guilt or innocence. For example, for the jurisdictions that punish murder under the “degree system” (first degree murder, second degree murder, etc.), the only way a person can be convicted of first degree murder is if the person had the willful, premeditated intent to kill another person. In a “degree system” jurisdiction, a person cannot be convicted of first degree murder if they did not kill a person without willful intent.