how this filter works

could somebody explain to me in simple terms how this ac filter works (attached)

how does it filter out higher frequencies?
i know the output signal decreases with increasing frequency but i just cant seem to grasp how it does it

It's a simple potential divider - at different frequencies the effective 'resistance' of the capacitor C will be different, it will decrease with increasing frequency. At the frequency where the impedance (AC resistance) of C equals that of R, the output will be half the input. Where it equals half that of R the output will be 1/3, and so on. It's a fairly crude filter, but it's commonly used - and it only has a very low roll off.

its a low-pass filter.

Caps appear as open-circuits to DC voltages, they also look like short-circuits to AC voltages.

You can appreaciate then that AC components will be shorted to GND thus not contributing to the output voltage.

the 3dB freq of the knee point of this filter is:

f = 1/(2*pi*R*C)

As mentioned it is a voltage divider its just the lower component is frequency dependant

For a resistor:
Z = R

For a cap
Z = j/wC - where j is the complex component and w is ang freq

So you can still do the usual maths that you would to work out the voltage of a resistive voltage divider just with j/wC (as long as you know complex maths...)