Quick Ratio - or acid test ratio - is used to evaluate company’s ability to overcome its short-term liabilities with its most liquid assets, like cash and cash equivalents, marketable securities and accounts receivable. It can be calculated by deducting inventories from the total short term assets, and dividing them by current liabilities.
Quick ratio is more conservative than its father - current ratio, because when calculating it, we should deduct inventories, which are not always so liquid. Inventory is excluded because some companies have difficulty turning their inventory into cash. It is because some companies are overestimating their ability to cash out their inventory, when thinking, that they are producing the best production that is on demand whenever they want to sell it. In real life it is different and usually it takes some time to realize inventory from the stock.