A future light cone is the part of space-time that can be affected by our actions in the present. Its boundaries are defined by the speed of light. If you imagine the Universe as having only two dimensions in space, then the area of space that you can affect 5 years in the future is a circle with a radius of 5 light-years; if you drew many such circles at different points in time, they would look like a cone. To affect a point in space outside your future light cone, you would have to send out some kind of order or projectile or information faster than the speed of light, and current physics says that this is impossible.