A number less than zero. Example: -0.25. Where a point on a line is labelled 0 and equally spaced points to one side of it are labelled -1, -2, -3 etc, these, and the numbers represented by points between them, are negative numbers.
Negative numbers can also be used to represent direction, for example when describing the velocity (speed and direction) of a projectile. If a ball hits a wall and is said to move at 20 metres per second and then -2 metres per second, this just means that at some point it moves in the opposite direction it started moving (i.e. it bounces back off the wall at a speed of 2 metres per second.