It's when you don't have the balance to cover the current leverage you have outstanding.
Let's say you bought Twitter for $44b but you didn't have $44b in cash so instead you said, here's $8b just put the rest against my Tesla shares that are worth more than that. Then those Tesla shares become less than that amount needed to cover that debt because of a draw down in the stock price. You get margin called, which is where the debtor liquidates all your shares to cover the debt unless you can provide more cash from somewhere fast.