Basically, a going concern means the business is still trading and generating revenue, rather than being in the process of winding down or liquidating its main assets. This doesn’t mean the business is necessarily profitable, just that the doors are still open for trade and there is no current intention or reason to shut the business.
This is generally quite an obvious fact about a business, but not always. A firm that was shuttered during the COVID lockdowns could be described as being a going concern if there is a genuine intention from the owners to reopen when the lockdown is lifted. In most cases, I think we would consider a business a going concern if it is currently trading, but clearly there are some grey areas on the fringes.