What does it mean to be a 501(c)(3) organization?
Being "501(c)(3)" means that a particular nonprofit organization has been approved by the Internal Revenue Service as a tax-exempt, charitable organization. "Charitable" is broadly defined as being established for purposes that are religious, educational, charitable, scientific, literary, testing for public safety, fostering of national or international amateur sports, or prevention of cruelty to animals and children.
Source: https://www.501c3.org/frequently-asked-questions/what-does-it-mean-to-be-a-501c3-organization/
What is a fiscal sponsorship?
Fiscal sponsorship is a formal arrangement in which a 501(c)(3) public charity sponsors a project that may lack exempt status. This alternative to starting your own nonprofit allows you to seek grants and solicit tax-deductible donations under your sponsor's exempt status.
Source: https://grantspace.org/resources/knowledge-base/fiscal-sponsorship/