Kamma, the word Kamma is often misunderstood, but it's all of your past actions, speech, and thoughts, that were intentionally done. So, for example, you're walking along the path and all of the sudden you hear "crunch" under your foot and you look back and you see one of the millipedes is now dying, half squished, because you accidentally stepped on it. A normal expression we say is, "You just killed that millipede", but did you intend to kill it? As I explained, you walked along and you didn't see it, you accidentally stepped on it, so you had no intention whatsoever to kill it. So your action was that you just killed the millipede, but you never had any intention to kill the millipede, therefore you made no bad Kamma at all. Somebody else walks along the path, sees the millipede and steps on it because they want to kill it. They not only killed the millipede, they had the intention to kill the millipede, that's bad Kamma. But if you actually didn't see the millipede, then accidentally stepped on it, it dies because you killed it but it's not bad Kamma because you had no intention of doing that.
So intention is very important when we look at our Kamma - did you intend to do that action, which then caused harm? In the same way, did you intend to do that action, that was of benefit to people? You won't get the same good Kamma out of that either if you had no intention to do it, you simply did something and all of a sudden a hundred people were saved. That's nice, but if you actually didn't have the intention to do it, and you didn't think you were helping anyone, then really, that's not the same Kamma as if you actually saw these hundred people and you knew that if you did a certain thing it would save all of them, and you intentionally tried to help them. So "intention", is very important.
Now there can also be shades of black, white, and grey here; intention isn't always perfectly understandable. Intention is not always something that's absolutely easy to see. There can be mixed intentions of both good and not good, at the same time.
So in the sense of the Buddha saying, "Kamma, I say is intention", what he's referring to is that your intentionally done actions, speech, and thoughts is what makes your Kamma.