What It Means
Upper circuit is the maximum price a stock is allowed to rise to within a session, based on its circuit limit percentage. Once hit, trading in that stock is automatically paused for the rest of that limit.
How It Works
When a stock hits its upper circuit, buy orders keep piling up at that price with essentially no sellers willing to trade — the stock is technically still "trading," but only in the sense that orders are queued, not actually matched. This most often happens on strong positive news (an unexpected order win, a favorable regulatory ruling) that draws in far more buyers than sellers at once. Getting out of a position you don't hold isn't the issue here — getting into one is: a trader trying to buy a stock already stuck at its upper circuit typically can't get filled until either the circuit limit resets the next session or the price is allowed to move further.
Warning
Placing a buy order once a stock has already hit its upper circuit often just adds you to a long, unfilled queue — don't assume you can chase a stock into a circuit and get a fill.