scalability is defined as the ability of an SEM product to adjust to increased input data while
continuing to produce timely and correct output.
in simple with the high load and long duration, how your app's will suport, that is if you are doing view a page with max load size 500 and may be 24 hours. to find out whether application suport or not.
Scalability testing is how well your overall architecture scales out when clustering or using other load-balancing techniques. Its the next step after basic performance testing. For instance, you might find that you can support 10,000 concurrent users per front-end web/application server. However, due to the single-database design of your system (or some other component that doesn't scale well) you find out that you can support no more than 5 front-end servers in your system. Thus, you've discovered that even though your software performs well (10,000 concurrent users on a server), it doesn't scale well. Google would be a good example of low-performance, high-scalability. They use massive amounts of cheap hardware to power their search system.