Pack drive is a term used to describe a dog's desire to be with and function as a unit with their "pack".
Like all traits it is a relative term that is hard to measure. In general it is used to describe dogs that are concerned about the whereabouts and activity of their owners, other dogs, and arguably livestock (in the case of flock guardian breeds that are raised with the livestock) in their "pack".
Dogs with "high pack drive" tend to follow around, or at least keep notice of the owners activity by choice ,without much formal training, and also tend to be concerned when family members depart. Dogs that are concerned about the safety of their family members can also be considered to have pronounced "pack drive".
On the opposite spectrum, a dog that seems to be more independent by nature, takes more effort to keep near the owners, and has seemingly less concern in general of the family unit has "low pack drive".
All dogs have some degree of pack drive and many factors such as age (puppies tend to be more needy), the early upbringing of the dog, the individual relationship the dog has with other "pack members", and other temperament traits may alter our perception of the genetic tendency of the individual dog.