In one sentence
Karma is action and the inevitable arc that follows from it — not punishment from outside, but the natural shape that any action takes in the world.
The depth — what it actually means
Most modern uses of "karma" mean cosmic justice or moral payback. The Gita's use is more precise and more useful. Karma is action — physical, verbal, mental. Every action enters the world and shapes what comes next. Some shapes are immediate; some take years; some take lifetimes. The Gita's instruction is not to fear karma but to understand that you are constantly making it. Every word, every choice, every glance is karma in motion.
Modern application — how to use this today
Audit your day at sunset. What did I send into the world today — through what I did, what I said, what I let myself think? You will be living inside the consequences of these for a long time. The Gita is not asking you to be perfect. It is asking you to notice that you are constantly sowing.