What is the law of karma?

It is the law of cause and effect. My guru, Sadhu Vaswani, referred to it as the law of the seed. As you sow, so shall you reap. You cannot sow thorns and reap apples.
The law of karma is universal; it applies equally to all. Every thought, word, deed, emotion, feeling and wish are seeds we sow in the field of life, In due course, the seeds will germinate and grow into trees, and yield fruit — bitter or sweet — which we shall have to eat. No one else can do that for us.

There are causes that produce their effect immediately. There are other causes that produce their effect after a long time. If you overeat, it is a cause you have created. It produces an immediate effect — indigestion. There are other causes which take time to produce its effect, every seed must yield its fruit. This is the law of karma.
Why our past karmas are kept a secret from us?

Don’t you think it is a great mercy of God that our past karmic links are not known to us? Else, it may be difficult for us to live in the world.