Net cash is cash and equivalents minus debt.
Net cash can be thought of as the amount of cash a company would have left if they paid off all of their debt.
To calculate it find cash and equivalents on the balance sheet, which is usually the first line on the balance sheet, and subtract from it all debt, both current and long term, which can be found under the liabilities section of the balance sheet.
Sometimes marketable securities are included in net cash, at the discretion of the person doing the calculation. But, care must be taken to avoid including securities with much risk. In general, it is best to completely avoid including long term marketable securities, and to only include current marketable securities when they are clearly identified and of extremely high quality, and even then leaving them out is acceptable.
Consideration is occasionally given for other elements of working capital, for example accounts receivable and accounts payable. But, including any elements from working capital beyond cash and debt raises the question of what not to include. There are other, better ways to analyze working capital, such as the current ratio and quick ratio. And, if you want to include all of a company's liabilities, then you may as well include all of its assets and look at book value, or to be more critical of questionable assets you could look at tangible book value.
It is generally safer to leave working capital out of the calculation of net cash.
Net cash can be subtracted from a company's market cap to calculate enterprise value, and some investors look for companies with near zero enterprise values and low cash burn rates as potential deep bargain investments, though such are rare to find without significant headwinds.
Net cash is also added to estimates of a company's value, if the estimator expects management to avoid inefficient uses of the cash such as overpriced acquisitions.