(a) Time is relative, meaning that it depends on the observer.
Suppose a clock is not moving. Then a person who is moving at a fraction of the speed of light sees a clock recording time more slowly than does a person who is at rest and who is looking at the same clock.
(b) Length is relative.
Suppose a stick measures one meter and is not moving. Then a person who is moving at a fractional of the speed of light sees the stick to be less than a meter. This is not an optical illusion -- it is a physical effect.
(c) No object can travel faster than the speed of light.
(d) Speeds do not add.
In the familiar world of slow-moving bodies, speeds seem to add and subtract. A baseball player who can throw a ball at 70 miles per hour is able to throw a ball at 120 miles per hour when standing in truck travelling at 50 miles per hour -- when the ball is thrown forward, the speeds of the truck and ball add. (If the ball were thrown out the back of the truck, then the speeds would subtract and a person on the side of the road would see the ball moving at 20 miles per hour). But the addition (or subtraction) of speeds fails at high velocities. A futuristic gun that can shoot a bullet at 70% of the speed of light when placed on a futuristic spacecraft travelling at 50% of the speed of light cannot fire bullets at 120% of the speed of light; the speed of the bullet and spacecraft do not add. The bullet ends up travelling less than the speed of light. In fact, when such a bullet is shot forward out of the spacecraft, it travels at 88.88...% of the speed of light (=eight-ninths of the speed of light).
(e) The speed of light is constant.
Independent of whether one is moving away or toward a light source, one measures the speed to light to be the same. Likewise, the speed of light does not depend or whether the light source is moving. This is an extreme example of effect (d). Instead, the motion of the source or the observer affects the color of the light. If the source and the observer are moving away from each other, then the frequency of the light decreases, which is known as a red shift because yellow light becomes more reddish. If the source and the observer are moving toward each other, then the frequency of the light increases, which is known as a blue shift because yellow light becomes more bluish.
(f) The time ordering of events is relative.
Suppose that one observer sees event A occurring before event B. Then it is possible that another observer who is moving with respect to the first observer to see event A occurring after event B.