A Doppler fetal heart rate monitor is a device used to detect the heart rate of a fetus without any invasive monitoring. The monitor looks like a little microphone and is pressed against the woman's abdomen. The monitor is moved around while the main device plays an audible signal that sounds a bit like you'd expect from a stethoscope (you've all played with a stethoscope, haven't you?? If not, ask to play with one the next time you go to the doctor. They're cool!)

The monitor is actually measuring the passage of blood in the fetus, and simulating an audible heart beat from the readings. I've decided a way to "spell" what this sounds like:

"ka-pshew, ka-pshew, ka-pshew..." (yeah, I'm a nerd. Get used to it. :P )

It's really awesome to hear, I think.

Here's a page on Wikipedia discussing in more deal what the Doppler fetal monitor does.