At Western universities like the University of Redlands, you're expected to acknowledge every contribution that did not originate from you such as ideas, computer code, and media. This approach is based on the Western idea of ownership such that if an idea is considered to be "owned" by Ana Solis, use of her ideas should acknowledge Ana Solis. However, when something is commonly known, believed, or accepted by your audience and not "owned" by anyone or anything (e.g. the sky is blue) you needn't cite it.
As a student, this can be tricky because you're still learning what's foundational, contested, and cutting edge in your discipline. Whether or not someone or something has staked ownership of an idea, concept, or technology may not be clear to you When in doubt, talk with your professor and err on the side of caution by citing a source.
For specifics on when to cite a source, read Warning: When You Must Cite.
To understand how to paraphrase without plagiarizing, read Fair Paraphrase.
