It's more of a general idea than a clear definition. I was limited by symbols already in the game as I did not want to add more just for this. The globe represents both exploration and (peaceful) expansion, so in the strengths section it can mean having space to expand or the ability to explore, and in challenges it usually refers to exploration goals or expansion goals that do not require (much) conquest. The golden age symbol is used to refer to golden ages and wonders.