A self-assertive person is ready, even eager, to say what they think, what they want, and what they will not accept. They do not wait to be invited. In training courses this is often treated as a good skill — speaking up without shouting. In ordinary talk it can lean a little the other way: a bit pushy, a bit much. It is not the same as being rude, and it is not the same as being selfish, though an unkind listener might hear it that way. You will meet it in psychology and HR language more than in the playground. Though actually, English already has assertive for almost the same idea; the extra self- just underlines that the confidence is about your own needs and opinions.