Kamma is intention. The results we experience depend on our intentions. That is why I think the Buddha said "Kamma is intention". Because you can perform the same two actions, but if the intentions are different then they are going to bring different results. So the Kammic effect that we receive depends on the intention behind our actions, speech and thoughts.
For example, perhaps you are walking along the path at night and your flashlight or torch starts to dim and you can't see very clearly. You don't see a living being on the path and you step on it accidentally. You did not intend to kill the living being so you don't get the Kamma of killing a living being. However, if you are walking along a path and you happen to see a spider and you don't like spiders, and you squash the spider, then you intended to kill a spider, so you get the Kammic result of killing a living being.
Even human law recognizes the difference in intention in assigning penalties to people. If there is a planned murder, premeditated murder, the result for that person and the prison sentence will be longer than, if a person kills somebody accidentally. So even in human law people recognize the difference in intention. Kamma is intention, this is what I believe the Buddha was trying to explain with that phrase.