Explained | What's the controversy around electoral bonds?

This year Rs 1,716 crore electoral bonds have been sold so far, versus Rs 1056 crore in 2018. But what exactly are electoral bonds and how does the identity of the bond owner remain a secret?

Does this mean government companies and foreign firms who are not allowed to contribute to political parties, can do so through these bonds? Could this lead to undue foreign influence in Indian elections? And what is the solution to all this?