Short-term debt is all interest-charging borrowing due within twelve months: credit lines, commercial paper, and the slice of long-term debt about to mature (which migrates here as its deadline approaches).
The warning to watch: short-term debt jumping without a matching rise in cash or operating cash flow. That suggests the business is living on credit it must constantly renew, and businesses that depend on rolling over short-term loans are at the lenders' mercy the day lenders get nervous.