What does accelerometer values represent

Well, presumably they represent some acceleration in each of the axis.
They don't mean anything without the context.

Usually an accelerometer would produce lots of readings ... perhaps these are mean accelerations. The instrument is usually measuring them much the same way a balance measures mass.

You can make an accelerometer by suspending a cork underwater by a thread attached to the bottom of the container. The angle of the thread from vertical is the acceleration, and the angle of the tilt on the horizontal is the direction.

Another way is to trap an air-bubble in a long glass tube (like a spirit level only longer) which is curved (concave-down). The position of the bubble is the acceleration.

Yet another way is just a weight on a lever mounted on a spring - this way the acceleration is opposite to the displacement of the weight.