Interoperability is the ability to interact and transact across different heterogenous blockchains. That means the protocol capacity should be able to interact between different blockchains and facilitate smart contracts between them.
Now the question is why it is essential -- The first objective is to eliminate the third party or intermediaries, the second objective is to improve performance and the third objective is to connect between private and public chains. The third objective is key for mass adoption also.
See when ever there will be enterprise level implementation of blockchain technology, then the businesses and enterprises will definitely adopt it but they will have their own protocol which will suit their business requirement. So it is apparent that there will be different use-cases and for different use-cases there will be different protocols. So for this ecosystem of blockchain technology to prosper in the next generation and to ensure mas adoption to be smooth, then it is essential that all these protocols should be able to interact and cooperate with each other. That is what interoperability is. That is why interoperability will be key for enterprise grade implementation and adoption of blockchain technology.
There is no doubt that those blockchains which have interoperability at their core will be more successful in coming days and will prosper in the blockchain sphere and also will survive the test of time.