The religion of Buddhism is about the pursuit of the Buddha. You must never stop looking for the Buddha, i.e., enlightenment and truth. If you think you've found the Buddha, you haven't. It represents an obstacle on the path to finding the Buddha. Specifically, if you find something that you spend your whole life looking for, you stop looking for it, right? In this instance, if you find the Buddha, you must kill it, because it will mean you stop seeking Buddha. And anything that stops you from seeking the Buddha is in fact, an obstacle. Even if you believe that to be the Buddha.
It's a journey not a destination. Anything that makes you stop on your journey is an obstacle.
That phrase brings back a lot of memories. First heard it in a game called "Way of the Samurai." Or it might have been the sequel. Can't recall. I found laughing and thought it was just Engrish at the time but no, that's pretty important to Buddhism actually.