Literally just a full swing around on the bar. It is the most common bars skill in gymnastics and appears twice in nearly every routine as gymnasts gear up for the dismount (as long as they’re not connecting directly into the dismount). Gymnasts can perform giants with legs together or straddled as they pass the low bar. The code doesn’t care.
You’ll note that the one-arm giant is also the same value as the regular giant (a B) despite being obviously more difficult cumbersome, which was done in an attempt to discourage female gymnasts from performing one-arm giants. It worked.