First, we money is a delusion we all agree to share - it has no intrinsic value whatsoever
Second, stock is an even more-removed delusion based on what people _think_ a company might be worth to buy - a value that changes at the drop of a hat (look at the recent massive dips, eg nvidia)
Third, and this is probably the real bummer - all this delusion is just going to make Smelly Tusk even more insufferable. Yay.
It's the notional value of multiplying the last sale price times the number of shares he has. He can't spend the money until he sells the shares, and there's no reason to think that they would actually all sell for the last selling price.
In fact, Musk is restricted from selling shares at the moment. His bank balance is the same today as it was yesterday.
(Caveat: he can borrow money today, using the shares as collateral. When the debt comes due, he can sell shares to sell it. So if he wanted additional spending money today, he could have it. Probably not up to the full trillion dollars, but some absurdly large sum.)
(Caveat bonus: he can actually roll over that borrowing indefinitely -- borrow more money to pay back the last one. Meaning that he's just acquired all of that money without having to pay a single dollar in taxes. It's good to be rich.)
