Differential Geometry/notation help

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Homework Statement



I have this theorem which I am having trouble understanding due to notation.

Theorem: Define S,S'\subseteqℝ3 to be surfaces. Let f:S→S' be a smooth map. f is a local isometry if for all p in S, and all w1,w2 in TpS,
<w1,w2>=<dfp(w1),dfp(w2)>.


The thing I don't get, is what does dfp(w1) mean?
What action is going on between dfp and w1?


Thank you for your time.
 
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df_p is the derived mapping from the tangent space at p in S to the tangent space at f(p) in S'.
 
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