Working capital
The money tied up in running the business day to day: inventory plus unpaid customer invoices, minus bills the company hasn't paid yet. When it grows, cash gets tied up; when it shrinks, cash is released.
Simple example
A shop holds $40 of stock and is owed $20 by customers, while it owes $25 to suppliers. Its working capital is $40 + $20 − $25 = $35.