Moving the goalposts is a rather straight forward fallacy to understand. Imagine yourself on an endless field with two literal goalposts that are constantly being moved.
At the beginning, someone tells you that you have been successful when you run between the goal posts. So, you start running. Then, as you’re approaching the goal, someone lifts up the posts and moves them further away… then further… and further…
You’ll never be successful, ever, because they are manipulating the criteria so that you’ll appear unsuccessful.
This is moving the goalposts.
Example of Moving the Goalposts
After Nathan and Steven both create their cityscapes, the boss says that while Nathan’s was more accurate, he’s giving Steve the promotion because Steve painted two of them instead of one.
Premise 1: The promotion should go to whoever paints the most accurate cityscape.
Premise 2: Steven painted two cityscapes instead of one.
Conclusion: The promotion should go to Steven.
Even if all (both) premises were true, it would still not make the conclusion true.