What's the meaning "A" out of "RGBA"

What’s the meaning “A” out of “RGBA” in the model of color?
And how does “A” with "R’.“G”.“B” compose to color.

A is alpha. It is normally used to set the level of transparency. In OpenGL it can also be used as a mask.

Extension: increasing values indicates more opacity. i.e. zero is completely transparent.