The subject of karma wasn’t taught extensively in itself by the Buddha primarily because it was well understood. This makes sense in the context of India 2,500 years ago where the subject of karma was already included in other religious belief systems, such as the Hinduism.
The word “karma” is somewhat misleading because it actually means “action”. Really what we’re talking about is the law of cause & effect. There are specific laws that relate to cause & effect and these are enumerated in Atisha’s ‘Lamp On The Path To Enlightenment’.
Unfortunately the law of cause & effect can be relegated and might seem like a basic subject, that is easily cast aside to focus on the more esoteric teachings. Actually the law of cause & effect underpins concepts throughout the Buddha’s teachings.
Of course, it’s not so much the law of cause & effect itself, rather the consequences actions. The law of cause & effect describes the mechanism by which actions later materialise into our day-to-day experiences. Again, this is elaborated upon in Atisha’s ‘Lamp On The Path To Enlightenment’.