Physicists say - speed of light is always the same regardless of our own speed - explain?

can someone explain the following illustration to me ? -------- 'When we follow the beam of light in a very fast rocket moving at 100,000 km per second. How fast is the light beam moving away from us? Common sense tells us that it is moving away from us 300,000 km minus 100,000 km per second. In other words, the light beam is 200,000 km per second faster than us. Wrong! Remember that the speed of light is always the same regardless of our own speed. From our rocket we would see that the beam of light is still moving away from us at 300,000 km per second! Likewise, if we were moving towards the beam at a very high speed we would still see the light coming at us at 300,000 km per second!' --- how is this possible ---?????