When a company has a successful exit, everyone gets paid roughly pro rata. When a company has a mediocre exit, the preferred stockholders get paid first — sometimes leaving very little for common.
Liquidation preference
Standard 'non-participating 1x preferred' means: if the company sells, preferred holders can either take their money back (1× their investment) or convert to common and share pro rata. They choose whichever is higher. In a strong exit, they convert; in a weak exit, they take their money back.
Participating preferred
'Participating preferred' is worse for common: preferred holders take their money back AND share pro rata in what's left. Less common today than in the 2000s, but still appears in down rounds and bridges.
Multiple preferences
Some down rounds carry 2× or 3× preferences. Each $1 invested is guaranteed $2 or $3 back before common sees anything. Stack up several rounds at 2× preference, and a $500M exit can pay $0 to employees.
If your company has done a big down-round recently, check the preference stack. If the preferences total more than 50% of the current valuation, your common stock is effectively worthless unless the company grows significantly from here.