When the central bank is "tightening", it slows the process of private bank issue by selling securities on the open market and pulling money (that could be loaned) out of the private banking sector. It reduces or increases the supply of short term government debt, and inversely increases or reduces the supply of lending funds and thereby the ability of private banks to issue new money through debt.
The operative notion of easy money is that the central bank creates new bank reserves (in the US known as "federal funds"), which let the banks lend out more money. These loans get spent, and the proceeds get deposited at other banks. Whatever is not required to be held as reserves is then lent out again, and through the magic of the "money multiplier", loans and bank deposits go up by many times the initial injection of reserves. (Wilkepedia.org.)
Accounts Payable: This is the most important source of short-term financing for many firms. Beware that increased use of Accounts payables (such as by not paying off when you should) can be expensive as most firms offer favorable terms for prompt payment and delaying payments can also upset your suppliers.