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These arrows indicate that Vin and Vout, respectively, are connected between the two circles (input and output). The bidirectional arrow is typically used in anglo-american based designs. In Europe it is much more comon to use a single arrow to indicate the (assumed) direction or polarity of a voltage (useful for setting up wquations for node analysis of a circuit).
These arrows are somewhat analogous to dimension lines on a mechanical drawing showing the distance between 2 points, only on a schematic the arrows & descriptors show the signals or power in some cases & the terminals.