please explain what control A do

hello
can somebody please give me a clear explanation on what exactly control A does? It says apply scale and rotation, but what does that mean?

Add a cube, go into object mode, press n to get the transform properties panel.
Scale the cube, rotate the cube, notice that the scale numbers and Rotxyz numbers change.
Then press ctrl+a. The scale numbers change to 1.000, the rotation numbers change to 0.000 and the dim numbers probably increase or decrease (depending on the scaling and rotations you did.) In a nutshell, that’s what it does, it resets those numbers to 1 and zero, respectively, and recalculates the size of the object’s bounding box based on the reset axes.

You probably read that and said to yourself, “What do I care about that nonsense?” Here’s why: when you deal with bones and armatures, sometimes you want to scale them. If the bone is already scaled, the scale you apply is multiplied by the scale it already has. This is not a problem if the scale it already has is 1, since that doesn’t change anything. But if that scale is 0.15, or 17, or something, suddenly your armature is flying all over the screen and you don’t know why. Same thing applies to rotation. You can try to rotate a bone 20 degrees or so, but Blender adds that to an existing 165 degree rotation, and finds it quicker to rotate in the opposite direction you intended to make that now 185 degree rotation. Again, your puppet is acting like its got a mind of its own, and you aren’t in control any more.

Those are the two gotchas I’m aware of, there are probably others, enough so that old timers, having been there, done that, routinely advise, “set scale and rotation, young grasshopper, or suffer the consequences…”

thank you , i have been wondering why its even useful