Janet asks;

what does this mean:
Products marked with *** dont exist in desired quantity in our stock.
You can buy them anyway and check the quantity we have in stock for immediate deliver in the checkout process.

This is a warning message to buyers to tell them that the amount of product they are trying to buy is not in stock. It looks like this:

An example of WHEN this shows is;

1. I have 10 DVDs of Courage Under Fire in stock:
2. A customer tries to buy 11 such DVDs

If you wish to actually STOP buyers checking out if you have insufficient stock, have a look in your Admin area, under Configuration > Stock. In here you will find some settings that can be changed appropriately. As an example, if I change "Allow Checkout" from true to false, that same checkout message that you asked about changes to;

As you can see, a different message based upon the setting used in your Admin area.

Conclusion

Exploration of the Admin area is an absolute MUST for anyone new to osCommerce. Without knowing what your store can do as standard, it's going to be a struggle for you to run – unless you want to pay someone to run it for you!