Exactly what means when you throw a pebble in the water of a lake. A wave appears, and you see that there are "valleys" and higher parts, that spread in circles. There where are valleys the level of the water is negative with respect to the original surface of the lake, and where there are higher parts the level is bigger (positive) than the original surface.
The only difference between the water waves and the light waves, is that in the light, what oscillate are the electric and magnetic field, see picture (electric field - blue, magnetic - pink). To see how these fields evolve in time see e.m. waves .
In the water, if you look at a fix point you see the water going up, reaching a maximal height, then going down, reaching the minimum (most negative level) and so on. With the electric (and magnetic) field in the light it goes the same - look at the figure in the e.m. waves . At a given point in space the electric and the magnetic field increases getting maximally positive, then decreases up to maximally negative, and so on. Just pay attention - the pictures for the e.m. field correspond to linear polarization. Sometime you will learn about circular and elliptic polarization.