An executive sponsor is a C-level administrator who has a vested interest in seeing a project to completion.
Ideally, the executive sponsor should be the highest-ranking manger possible, relative to the size of the project. Successful sponsorship requires a deep understanding of organizational culture and awareness of how the project will help the organization achieve its goals. Part of the sponsor's job is to promote the project within the organization, making sure that everyone understands the benefits the project will provide.
In some organizational cultures, the executive sponsor is the person who is responsible for financially authorizing a project and creating the project charter. The sponsor should work closely with the project manager (PM), but is probably not the project manager's boss. Typically, the PM is responsible for day-to-day execution and the sponsor only steps in to make decisions when change requests might affect the project's scope.