The only way to explain the gap is honestly. However, you don't need to make the two-year gap sound like a failure. I'm sure that during those two years you learned many things.
I suggest that you give some serious thought to what are the three most important things you learned during those two years.
https://woollymammothwoolencompany.com/ Then you can present an answer like, "I took two years off to try to prepare for the IIT-JEE, but was unsuccessful in achieving that goal. It taught me that one must pursue a goal for the sake of the learning along the way, not just for achieving the goal…" An answer like that, showing the wisdom that you gained in those two years, is probably the best thing that you can do, since the failure is obvious.