I’ve read about this, but I’ve never understood what that means. I can click my tongue but I can never hear any kind of echo. Is it a very particular click?

You will not distinguish an echo, just a change in the "tone" of the sound you make as it reverberates in your surroundings.

The echo pulses are there but just milliseconds after the sound you made. It will all blur into a softer or harder reverb depending on your surroundings.

To hear an actual separate echo pulse, it would need to be 20-100 milliseconds later than the original sound. That corresponds to a large hall tens of meters in size.

Doing a loud clap with your palms aligned and as flat as possible produces an adequate sound.

Sharp and short, alternating left and right at about 4 Hz, on the side of the tongue back by the molars. Feels like pulling the side of the tongue away from the teeth. It’s a much clickier click than a front of the tongue ‘click’.

See the videos in my other comment. Daniel Kish explains how he does it.