At the time of software testing using the integration technique, the functionality of the software being tested, is divided into independent modules. Top down is that approach which employs testing starting from the highest or main module and proceeds downwards to the lower or sub modules.
One of the chief features of top down approach is the use of under developed temporary modules called STUBS. If we suppose a software functionality needs to be tested with one main module A in combination with two to three sub modules (numbered 2&3), then we say that the testing will start from the module A and will proceed step by step towards testing of communication between sub modules 2 and 3. In addition to this we simulate the behaviour of another combination of sub modules called stubs which have been developed for testing at a level lower than sub modules 2 and 3. Likewise we proceed further down to testing of sub modules present lower down the pecking order.
To conclude we can say that testing using top down approach allows us to create a sturdy and uniform framework for the software product being tested before its eventual release.