In very simple terms:
"Liquidation Preference" is an agreement between a company and an investor that when the company is acquired or IPOs, the company will pay the investor some specific amount of money BEFORE any other shareholders get paid. If the company negotiated the funding well, the liquidation preference might be 1x (basically saying the company promises to pay back, in full, the investor's original investment if the company is ever sold)
Things get scary when the liquidation preference creeps up to 2x, 3x, or higher. It's possible to raise $5mm with a 3x liquidation preference... which means that the investor will get $15mm payout BEFORE any other shareholders in the event of an exit (which means that, after raising that $5mm, the company can not have an exit less than $15mm without 100% all the proceeds going to the original investors).
Raising $5mm with a 3x liquidation preference, then selling your company for $15mm, is an easy example of how (even founders) can walk away with $0 after a $15mm sale.