What does it mean when someone says that the gravitational fields of heavy bodies is a "negative energy?

You need to understand potential energy to understand the answer to this very good question.

it all boils down to distance from a planet (Gravitational Potential Energy = mgh)

Gravity is typically negative, right? Because you are usually going down. I say this for obvious simplicity. Gravity in almost all systems is considered negative for simplicity (the ground is down and the earth pulls you down, which is negative from up)

All right. So that's settled.

Now you know Force = G(m1m2)/r

cool. That's the attractive force between any two objects.

Well Potential Energy (represented mostly by the capital letter U)

is an energy and energy involves work.

it is also mgh. The h is the distance from the earth (don't forget that all objects are related to equations by their center of mass, so h is the distance from the center of the earth. r + h)

great!

so the work (energy) required to oppose the gravitational energy (the acceleration of gravity towards the earth) in order to get an object (mg) to some height (h) above the earth (Potential Energy = U = mgh) becomes negative strictly because gravity is assumed to act at an infinite distance from any object.

So an object trapped by a large body, and which has no ability to escape that body's gravitational pull, is having NEGATIVE work done on it. This is because POSITIVE work would have to be performed on the object in order for it to reach the escape velocity of the planet and leave the gravitational pull.

thank you bye

It means that whoever is saying that has NO CLUE what he/she is saying.