The phrase originates from the image of someone literally being stuck up a tree, unable to climb down or go any further.
Here are some examples of how it's used:
* "I'm up a stump on this math problem. I don't even know where to start."
* "We're up a stump trying to find a new apartment in this crazy market."
* "The project is up a stump because we're waiting on materials to arrive."
It's a common phrase that can be used in a variety of situations to convey a sense of being blocked or unsure.