When someone is negligent, he or she does something or behaves in a way that is unreasonable. For example, if I leave a saw blade in my front yard while children are playing in the yard, most of us would agree that I should not do that. Doing so would be unreasonable. Most of you would require that I put the saw blade in a place that is not accessible to children. This would be reasonable.
If a child then picked up the saw blade and cut herself, I could be liable for her damages because I was negligent.