Simple Armature pose problem (I think)

I am fairly new to Blender but understand most of the basics. But I cannot understand why the arm mesh gets distorted in attached picture.

I used ‘Apply bone heat weights’ to create vertex groups for armature. I am not quite sure whether to work on mesh, armature, or vertex groups and weight painting.
Can someone point me in the right direction. Appreciate any comments from the many experts here.

I have heard good things about bone heat, but have never used it myself.

I personally recommend assigning vertex groups to each bone, then weight painting to fine tune the result.

Thanks Ill give that a try.

I am guessing that you cannot use an armature until you apply subsurf. The below result is after I applied subsurf and the redid the armature with bone heat weights