First of all, i want to say that "SHOW STOPPERS" bug vary from product to product or from s/w to s/w and it all depends on what time the issue/bug is reproduced. For explaination, i would give you a simple example of this 1. Suppose you type yahoo.co.in and try to login with your login id and pwd and the login page does not appear after entering the correct information, then it would be a SHOW STOPPER bug
2. Lets assume that you are logged in successfully and now if you try to logout from the yahoo mail application and its not working means you are not able to logout, then also it would be a "SHOW STOPPER" issue/bug.
The above mentioned all the examples are general examples for understanding purpose only. Hope this helps you