In order to appreciate the very strange nature of ultra-compact objects, first we need to consider two concepts. First is how fast you need to go to escape the planet you're standing on.
You already know that you can't jump off the earth. And you also know that the faster you jump the higher you go and the longer it takes gravity to pull you back to earth. If you were to jump with a speed of 7 miles per second, you would jump off the earth and earth's gravity could never pull you down--we call this special speed escape velocity. For earth's mass and size, the escape velocity is 7 miles per second.