FRANKENSTEIN54 said:
Well , the bullet or the light doesn't change their speed , whether I'm moving or not .
OK, so this statement is false, but part of the reason that it is false is terminology. Physicists use words with very specific technical meanings.
Two important technical terms are "coordinates" and "inertial frame". Coordinates are a set of labels that we use, a kind of address book for locating events in space and time. We use coordinates as a mental tool to make calculations easier. An inertial frame is a particular type of coordinates where the laws of physics are particularly simple to calculate because a lot of things (Christoffel symbols) are zero.
You have an intuitive picture that a bullet is something in the real world travelling on a real path doing its own real thing. That real path exists in the real world, and it doesn't depend on our mental model of it. In particular, its real path doesn't depend on our coordinates. We can imagine all sorts of things about it, but none of our imagination changes any real thing about the bullet. That is correct, but that is not what the word "speed" refers to.
The concept that you are thinking about is called the four-velocity. The four-velocity is a geometric quantity that describes the motion of the bullet and it does not depend on our imagined coordinates. Different people can imagine different coordinates, and they will nevertheless agree on the geometric four-velocity. They will say that it has different "components" with respect to their chosen coordinates, but they will agree that the different components are just their different labels for the same geometric thing.
Speed is a different thing. Speed is relative, you must always specify the thing that you are measuring the speed relative to. Geometrically, speed is a kind of "angle" (in a technical sense it is an angle in time rather than in space) between the four-velocity of the object whose speed is being measured and the four-velocity of the reference relative to which it is being measured. Usually, the thing that is used as a reference is a coordinate system, and often specifically an inertial frame. You cannot measure an angle without defining the two sides between which the angle is measured. Same with speed, the reference is part of the specification of speed.
Now, finally we come back around to your statement. You said "the bullet or the light doesn't change their speed , whether I'm moving or not". So here, you have a description of some speeds, the speed of the bullet relative to moving you and the speed of the bullet relative to stationary you. These two speeds are different. You also have a description of the speed of the light relative to moving you and the speed of the light relative to stationary you. These two speeds are the same.
So "the light doesn't change their speed, whether I'm moving or not" is correct. But "the bullet doesn't change their speed, whether I'm moving or not" is wrong. It does. Speed is relative. It is the four-velocity that is the relevant non-relative geometric quantity. Everything has its own four-velocity that is what it is regardless of the reference, whether it is a bullet or light. But the speed of light is different from the speed of a bullet in that the speed of light is the same for all references.