You can use the Gradient panel (Window > Gradient) or the Gradient tool to apply, create, and modify gradients.
Gradient colors are defined by a series of stops along the gradient slider. A stop marks the point at which a gradient changes from one color to the next, and is identified by a square below the gradient slider. The squares display the color currently assigned to each gradient stop. With a radial gradient, the leftmost gradient stop defines the center point’s color fill, which radiates outward to the color of the rightmost gradient stop.
Using the options in the Gradient panel or with the Gradient tool, you can specify the number and location of stops, angle in which the colors display, aspect ratio of an elliptical gradient, and the opacity of each color.