A footbag or foot sack is a small, round bag filled with dry grain (e.g. rice) or sand, which is kicked into the air as part of a competitive game or as a display of dexterity.
“Hacky Sack” is the name of a brand of footbag popular in the 1970s (currently owned by Wham-O), which has since become a generic trademark.
Hacky sack games are similar to earlier Asian shuttlecock sports such as jianzi. The most common game of hacky sack consists of two or more players standing in a circle and trying to keep the sack off the ground for as long as possible. You could also challenge yourself to try to keep the sack off the ground for longer and longer each try.