No, it is what the shareholder can, in principle, "take out" of the company, appropriately discounted. For a going concern, without excess cash, that is going to be, to a first approximation, something like discounted earnings.
> So if a lemonade stand has 3bn in cash it's done pretty well selling lemonade surely?
Not necessarily. When starting a business you need cash, even pre-revenue. Even if the cash were generated entirely from the business somehow the business endeavor may no longer be valuable. Maybe people don't like lemonade anymore. Maybe lemons have gone extinct. In those cases future earnings are zero.
If someone wanted to acquire the lemonade business what would they pay? Probably not $3bn because that would just be moving numbers around (there is no reason normally to buy cash). They would pay whatever they could get out of the lemonade business proper (assets, future earnings). The $3bn would be returned to the shareholders.