This is the anatomical position.
The anatomical position is the position from which all movements are described.
Think of standing upright feet together and facing forwards, arms by your sides with your palms facing forwards.
The red line is the midline.
This is important for terms like
medial – this applies to the side of a limb that is nearest the midline
lateral – this applies to the side of a limb that is furthest away from the midline
It does not matter what position your arm is in – the term always applies to that side of the limb.
For example (and hopefully not to confuse you):
If you were to turn your arms around so that the palms of your hands were facing backwards -
the lateral side of your forearm would now be nearest the body but it is still called the lateral side of your arm.
To make it easier to understand – imagine you painted your arm green and blue as in the diagram.
No matter what position you put your arm – in the air, behind your back or under your knees! – the blue side would still be blue etc.
Just so with medial and lateral – it doesn’t change.
It is described and named as it is in the Anatomical Position.
I hope that makes sense now.