It is called a slope. A slope pointing downwards like this slash \ is considered a negative slope. If a slope pointing upward like this slash / then it is considered a positive slope. What if you had a line on a graph not pointing in either of those directions? If it were pointing straight up like this, | then it would be known as an undefined slope. Last but not least, if you had a line on a graph straight across like this, ____ then it would be zero slope :D
Hope this helps and is a little clear to understand :)