A work for hire is an exception to the general rule that the person who creates a work is the legally recognized author of that work. A work for hire agreement is when for a flat fee, you give up all ownership and administration rights for your works for the life of the copyright.
Under US copyright law and in some other jurisdictions, if a work is "made for hire", the employer, not the employee (composer) is considered the legal author. This means that the employee is not entitled to collect publishing on the work despite the fact that they were the composer.
It is important to be well informed when entering a work for hire agreement, and to consult with legal council when deemed necessary.