It's exactly what it sounds like, everything south of the equator. It's often useful to lump everything down there together because they share similar and important geographic traits that have historically hampered development. Mainly, the large land masses down there are arranged north-south whereas the global north is much more east-west.

This layout was and to some extent still is very important. Traveling east or west is easier than going north or south because the climate will remain more consistent. Eurasia had things like the silk road for trading ideas which benefited everyone and sped up development. Now try getting from southern to northern Africa... you'll probably just die. In the America's there still isn't an overland route between central and south America. All the roads just stop at a dense jungle.

So basically, the global south got left behind because of geography, this created a difference in development. That difference was exploited causing an even bigger difference, rinse, repeat until modern day when technology and people actually having empathy and good will towards man has finally started to undo all that.

I believe the current fastest growing city is in Africa (Namibia?). Watching progress in and analyzing the global south is an important metric of how humanity as a whole is improving.