FDA ODAC Meeting Material discussion analysis, page-187

My understanding is that what they are asking for is a metric and test framework for assessing the quality of each batch of product to ensure that once it is a commercial product, the result for patients is the same as the result of the trial. That paragraph was saying that "how it works" hasn't been adequately demonstrated, and knowing how it works in fine detail allows you to be able to do a quick test on a batch of product and say "yes, this one will work just as good as the trials showed us it would, but this other one here is no good and won't have any effect". That's why the document talks a lot about potency etc.

efficacy = does the product work? (YES!)
mechanism of action = how it works? (need more info)

From the extra information, they want to be able to recommend a benchmark or some standards for regularly testing the product in the marketplace. With most drugs there are existing standards for this kind of thing that can be reused or adapted (like does pill X contain at least YY milligrams of compound Z? because we know that compound Z is what cures headaches/itchy toe/mad FOMO) but for cells this is tricky, because they're not chemicals so you can't pop it in a machine and do a spectrum analysis or whatever they do for their QA usually (I'm not a chemist). They're right to ask the question, and it's something that comes up on these boards from time to time as a point of interest for both bulls and bears on the share. The posters above talking about risk/reward assessment have kind of covered this already, I'd recommend reading what they have to say, though there's only 30 trading minutes before you'll either be very very happy or very very sad, so read fast!

This is my assessment of that bit of the document, I have a scientific background, but not in chemistry or medicine, so google some terms and DYOR.