where is this equation from?
what does it equal?
Well, presumably you're talking r as an electrical resistance, and c as an electrical capacitance. (Note that people ordinarily use the capital letters R and C for these quantities.) Also, you're not describing an equation, you're describing an expression.
The expression RC has units of seconds. It is known as the "time constant" of a circuit formed by a resistor and capacitor. Imagine giving this circuit a step input, perhaps from 0V to 1V. The time constant is the length of time needed for the circuit's step response to be within a factor of 1/e of its final value. The factor 1/e is around 37%. Thus, after one time constant, the output voltage is about 63% of its final value.
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