Just as colon the organ, um, moves things along, so does colon the punctuation mark. It signals to a reader that what follows is a list, an elaboration or definition, or something else that continues the thought. It says that what follows is so closely related to the first part of the sentence that a period is too abrupt, but not so close as a comma would indicate. In a driving metaphor, think of a colon as a tap on the brake before moving the car into a curve.
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