Explanation

Linear gradients are defined by a 'linearGradient' element. 'x1', 'y1', 'x2', 'y2' define a gradient vector for the linear gradient. This gradient vector provides starting and ending points onto which the gradient stops are mapped. The values of x1, y1, x2, y2 can be either numbers or percentages.

Further information:

http://www.w3.org/TR/SVG11/pservers.html#LinearGradients