Jell-O is a sweet dish, similar to (but not exactly like) British jelly. It has a sort of very soft, rubbery texture, which is hard to explain, but you could probably manage to walk through it, if you had enough of it to walk through. However, it would be very difficult and you would have to take it very slowly.
The author is saying that life is like trying to walk when the air is made of Jell-O. It's possible, but it's a real struggle.
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