what is Tau in this very simple circuit?
After a "long time" the capacitor will charge to a voltage that is slightly less than 12V. This voltage is approximately 10.909V and this value is determined solely by the potential divider formed by R2 and R1.

It rises exponentially towards this voltage at a rate governed by both resistors and the capacitor. If it were just a single resistor (R1) with no parallel resistor (R2), \$\tau\$ would be \$C_1\cdot R_1\$. But it isn't; R1 is effectively reduced in value by the effect of R2.

This new value of \$R\$ is the parallel combination of \$R_1\$ and \$R_2\$:

$$\tau = C_1 \frac{R_1 \cdot R_2}{R_1 + R_2}$$

Notice that R is the product/sum of the individual resistors R1 and R2.

Another way of looking at it is converting V1, R1 and R2 to its Thevenin equivalent. It might be worth googling that if you weren't aware of it.