It changes the temporal interpolation to "Continuous Bezier". This means that at this keyframe no abrupt speed changes will happen. This is best visible in the speed graph: "Continuous Bezier" will make sure that if you move the left handle of a key up or down (i.e. change the speed with which the move arrives at this point) the right half will move with it, too (and vice versa). Without "continuous" you can move the left and right half up and down independently, which means you could arrive at the key with 30px per second and then leave with 100px per second, for example.