Some applications, like Google Earth, use KML data to represent map routes. There is an easy link now, in My Maps, to click and open the map directly in Google Earth. But if you need the KML data for importing elsewhere, you can save it this way:
First, in one of your maps in Google My Maps, copy the URL for "View In Google Earth" by right clicking the link and copying the URL. On a mac, it's "copy link location".
Then in a blank browser window, enter the URL in the address bar:
Now change the output by switching output=nl to output=kml. Entering the URL will give an option to save. Save the KML file somewhere on your drive, and then you can import it elsewhere.