Cub scout rank levels are equivalent to the dens group by grade level, Lion (KG), Tiger (1), Wolf (2), Bear (3), Webelos (4), and Arrow of Light (5). As a scout progresses through each rank, they work on adventures to earn their rank badges. Advancing through the ranks becomes more challenging as they are designed to be age-appropriate for the skills of the scouts as they grow.
Cub Scouts complete Adventures that are specific to their grade and rank. A number of Adventures must be completed to earn the badge of rank for each grade level. Completion of Adventures is how the aims of character, citizenship, leadership, and personal fitness are developed. A scout must complete 6 required adventures and 2 or more elective adventures to earn their rank badge.
For details on the adventures for each rank, see Cub Scout Adventures | Scouting America