What is the meaning of the phrase serve at the pleasure of the president of the US?

From all I have read, it seems that those who "server at the pleasure of a leader" are paid employees that can be terminated at will and without just cause. The termination cannot be the result of illicit reasons like discrimination. This still allows for collorbration and decenting opinions to be shared respectfully with the leader but when the final decision is made by the leader, the employee has a choice to comply with the leaders's direction or not, and opposing actions my result in termination.

I would welcome other definitions but this is how I understand it.