Astigmatism
Astigmatism means that the eye instead of focussing light at one point on the retina focuses it at two separate points. These points may be in front or behind of the retina leading to blur. Sometimes we describe it as the eye being shaped like a rugby ball instead of a football. A lens must be used the brings the two foci onto the same point on the retina. In your prescription this is the second number with a minus sign in front of it. The number after this - the axis, tells us the direction the distortion is occurring in.
Astigmatism is caused by light being focussed at two points instead of one, a lens to bring the two foci together must be used