The simplistic explanation is that a thrust of 115,000 lbs is just enough to support a weight of 115,000 lbs (including its own weight) directly against the pull of gravity.
Place 96,640 lbs of, let's say, iron weights on a platform. Attach to this "payload," with 100 lbs of cables, the GE 90-115b, pointed straight up like a space rocket. Since the engine itself weighs 18,260 lbs, and the cables weigh 100 lbs., the total weight of the engine, cables and weights is 115,000 lbs.
Run the engine up to maximum throttle, and it will just exactly support the weight of itself and the extra weights and cables (115,000 lbs total) against the pull of gravity. You can remove the supporting platform and the engine will hover, holding up the total weight - without descending, but also without climbing.
Add 1 pound to the weights to create a total weight of 115,001 lbs, and the whole thing will sink slowly - the weight is greater than the thrust. Reduce the total weight by 1 lb to 114,999 lbs and it will rise slowly under the engine's "excess" thrust of one pound.
Now, that is just a thought experiment in an "ideal" world. As tdracer points out, in the real world you'd run into all the losses he mentions. In addition, modern computer-controlled (FADEC) engines are often artificially limited to a specified thrust by software that limits rpms and fuel flow.
You'll also note that - thanks to the magical device called a "wing" - the 775,000 lbs of a 777-300ER can be "flown" with only 230,000 lbs of engine thrust. Unlike our thought experiment, the engine thrust does not have to support the vertical weight of the plane directly. It simply has to move the wing fast enough horizontally to provide "lift" that equals or exceeds 775,000 lbs.