Luring is using a high value item (usually food but sometimes a toy), for the dog to follow to shape a behaviour.
Once the dog is following the lure into the correct position consistently a command can then be paired with the behaviour.
This is using positive reinforcement to shape behaviours. It is particularly useful in shaping behaviours with puppies as they are a blank canvas, but it can be used with dogs of any age. Luring builds engagement between you and your dog as following you with a high value item makes you more interesting and fun.
This is Lennox learning a focused heel at just 10 weeks old using a food lure. At present he is unaware of what he is doing but the behaviour is being conditioned subconsciously for later life.