What do you mean by -3db cut off frequency in low-pass-filters?

I have a low pass filter as follows.

^{simulate this circuit – Schematic created using CircuitLab}

Using voltage divider I get

$$frac{V_{out}}{V_{in}} = frac{1}{sqrt{1+(omega RC)^2}} $$

But now I got a problem. I have been given the property of low pass filter as -3db cut off frequency. How am I supposed to put the value of $omega$ if no frequency is given but only -3db?

If It helps the reader to understand more the question: Actually I have been given two resistors with 1kΩ and 5kΩ and two capacitors with 2nF and 4nF and I have to choose only three of the component, to realize the circuit with -3db cut of frequency. So I came up with this schematic that could also be the possibility.