How should one interpret a requirement that a proposal in pure math be "jargon-free?"
I think that the key idea here is that they want to know why you are doing what you are doing more than what you are doing.  

Let's say you're diligently working towards establishing an isomorphism between soliton elves and the christmas-tree singularity.  That's all well and good, but:


There are lots of properties in the world.  Why is isomorphism between soliton elves and the christmas-tree singularity an interesting property to try to establish?
Is there a higher-level strategy to your approach than "just do lots of math until the problem is solved"?
Why is the approach that you are taking a good one?
If you establish this property, what are the consequences, and what comes next?


A really good and clearly written proposal should be able to have its logic grasped by pretty much any scientist, whether or not they are capable of judging its originality, plausibility, and significance in detail.