Assuming that JFK is the first airport in the US that you land in on the way from ISB, you will have to go through the following when you land there:

1. US immigration
2. collect checked baggage
3. US customs
4. recheck baggage
5. go through security screening

Step 2 is required in case US customs wants to look at something in your baggage. Step 5 is required because you could theoretically have moved prohibited-in-carryon items from your checked baggage to your carryon baggage.

Allow sufficient time for all of the above steps in your connection, especially if it is not through-ticketed.