Functions are the cornerstone of calculus, acting as a broker between two variables to create a dependency. Understanding functions, their graphs, and how they relate input to output is the highest leverage skill for mastering calculus. The directionality of this relationship, where each input yields a unique output, is fundamental.

Impact: High. Establishes the primary building block for calculus, framing its importance and core mechanics.

In the source video, this keypoint occurs from 00:00:31 to 00:05:36.

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