Blocking or Cease and Desist for Dogs

Blocking usually occurs when a dog is unhappy with the behaviour of one or more other dogs - usually dogs who are playing roughly or look like they might
get into a fight. It can also happen when a group of dogs are doing a task, such as hunting, and a dog decides that one or more dogs are paying too much attention to something off task.

The blocking dog will walk between the two dogs (or a dog and the inappropriate object of attention.) The other dogs recognize this as a sign to cease and desist and go back to more appropriate behaviour.

Humans can do this as well, and it works well when a dog comes up and is over
enthusiastic about greeting your dog - i.e. won't stop sniffing. Even if one of the dogs has poor social skills and doesn't get what you are saying, the other dog is likely to feel like you are looking out for it (particularly important for your dog, who should feel like you will keep it safe!)

Blocking is not appropriate for breaking up a fight, but is appropriate when dogs are just starting to get overly amped up.  (For information on what to do and not do about fighting dogs check out Terrierman's (P. Burns)
article Knowing What NOT To Do in a Dogfight)