This describes a posture (shaped like a capital "A") where the arms are held slightly away from the sides of the body with the arms and forearms in line with each other, similar to an informal curtsey but without the need to bend the knees or hold a skirt. This is more relaxed than the close to perpendicular T-pose or outstretched arms, but less relaxed than arms at sides where the arms simply hang against the torso due to gravity.
You often see this pose in a few contexts, for example:
a character may be showing off or evaluating an outfit.
a character may be posed similar to a Sailor Moon magical-girl style transformation scene or the like where the character appears to be relaxed, but floating.
The name comes from 3D modelling as an alternative to the more common T-pose. The A-pose allows for the shoulders to be modeled and textured in a more natural position, and the defined angle can aid in rigging.
