Glo Up is a term coined by Chief Keef, founder of the hip hop group known as Glo Gang. Glo Gang was formerly known as Glory Boys. Glo Up means multiple things, but it's mostly used as 'Glow Up'. Glo Up is also a believe to be a standing pose, this belief started after Chief Keef posted a picture on his instagram with his legs spread and hands crossed in front of him, the picture was titled Glo Up.
The term Glo Up is used in many of Chief Keef's songs, as well as songs by members of Glo Gang. The term vary by usage, for instance, in Chief Keef's 'Ight Doe' song, he says "Got this 30 on me, it's mighty long - F*ck with Chief Keef, b*tch you might glo". Chief Keef is saying anyone messing with him might get turned into an angel, angels have a hollow around their head which glows, hence the the reason they'll glo.