Cash out refinancing (in the case of real property) occurs when a loan is taken out on property already owned in an amount above the cost of transaction, payoff of existing liens, and related expenses. Strictly speaking, all refinancing of debt is "cash-out," when funds retrieved are utilized for anything other than repaying an existing loan.
In the common usage of the term, cash out refinancing occurs when equity is liquidated from a property above and beyond sum of the payoff of existing loans held in lien on the property. This is often done when the value of the property has increased allowing the mortgagor to increase their mortgage taking cash out to spend on other things.