What do these values in the freeverb diagram mean?
It's a Lowpass-Feedback Comb Filter, so let's dissect this one at a time:


Comb filter: You have delay line and add the output back to the original signal. That gives you a single echo
Feedback: You take some of the output of the delay line and put it back into the input of the delay line. That gives you a series of echo's that repeat regularly. You need to put a gain &lt; 1 on this, otherwise it will repeat forever. That gain is called the feedback gain and controls how fast (or slow) the echoes decay.
You put a lowpass into the feedback path to model the typical absorption in a room that's higher at high frequencies and lower at low frequencies. You basically make the feedback gain frequency dependent.


$d$ is the pole location of the lowpass filter. That determines the cutoff frequency of your lowpass filter

$f$ is the feedback gain and determines how quickly the repeating echoes decay. That should be a function of the reverb time (at low frequencies) and the delay size