Working line dogs are bred from dogs who have titled in venues that test their physical ability. Examples of activities/sports/venues that working line dogs partake in: bite sports (IPO, French ring, etc.), herding, tracking, agility, etc. The dog's mental and athletic abilities are tested to the extremes so that very specific traits (ex. how fast, how fixated on certain things, how hard they bite, how long they can go without a break) are preserved and/or magnified.
Show line dogs are bred from dogs who have titled in conformation (structure, gait, etc.). Things like the angulation of their joints, their teeth/bite, how they move, their behavior while being handled and prodded at by strange people, etc. are prioritized.
Working line dogs can succeed in the show ring. Show line dogs can succeed in sports. But typically and -very generally- speaking, show line breeders who focus solely on conformation do not care as much about how driven a dog is to bite, fetch, herd, etc. And working line breeders may not care about X percentage of white on the coat that may disqualify the dog from the show ring, etc. This is why it is common for people to say "show line dogs make better pets". The idea is by focusing on conformation, the working ability of the dog has not been bred for and may be watered down. However, there are overlaps in goals between both worlds. For example, a dog with poor conformation (ex. bad hips) would not make a good working dog because poor structure or pain affect the dog's speed and agility. And a dog that has poor working abilities may be held back by poor structure or mental ability (which would cause it to fail in the show ring too).
But like I said, this is a very general answer. There are absolutely breeders who dabble on both ends of the spectrum. And a dog's personality is not solely defined by whether it is show or working line. However, generally speaking I do believe that the average pet owner (ex. I want a dog who is reasonably calm and cuddly, can tolerate 1-2 walks a day, a family dog who will chill in the house or hang out in the yard, hang out at the kids' baseball games, don't want to invest too much in training, etc.) should steer clear of working lines - of any breed.