Breeders terms usually means placing a dog out with someone else with a view of breeding with it later.
Only the registered breeder can breed with the dog, not the co-owner.
Often a dog placed on terms is one the breeder thinks will be valuable to their breeding program down the track, this gives the dog a good family home but allows the breeder to have access to the dog when and if needed. Of course that is if the dogs are of good conformation, great temperament and pass all health testing. Why you ask, it boils down to we can't keep them all - Breeders get the best of both worlds, access to their pedigree dogs and the dogs get a loving home.
If it's a bitch, the breeder will likely want 2 to 3 litters from the bitch and may insist the bitch come back to them for the litter. You will need to make the bitch available at varies stages leading up the mating and for up to a week prior to whelping and up to 8 weeks after whelping.
Male dogs on terms are not as complicated, the bitch usually comes to the dog and you do very little except make the dog available to the breeder, when needed. Timing is critical when a bitch is in season, bear this in mind when agreeing to taking a male dog on terms as the dog will need to stay entire.
Be sure you want to go down this track, "Breeder Terms" if done incorrectly can turn into a disaster, get everything in writing first and fore most.