A Jumbo set is an offensive formation that is usually a short yardage/goal line formation where extra TEs or Lineman are brought in to beef up the front line.
A Heavy set is another name for this type of formation.
The graphic showcases two of a number of Jumbo/Heavy Sets that an offense can utilize. The first is an Ace/singleback formation, that incorporates an unbalanced line and 4 TEs. The second set is from the I Formation and utilizes 3 TEs.