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A yoyo is a toy. It is a round object with a string around its middle; the string is wound round the object, which is then allowed to drop downwards. When is reaches the end of the string it rebounds so that the string winds up again.
This link has many images of yoyos: yoyo - Google Image Search
The song is saying that metaphorically "I" am held and am being played with like a yoyo - that "I" cannot escape, and that "you" control my actions.
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