What does "he was due to be put down" mean in the sentence below?
His new foster family (it's about a dog, not a person) saved him from a rescue center where "he was due to be put down".
Does it mean
where he supposed to keep there
or
where committed his paper works (I mean registration of him or something)

Question

Answer

The term "put down" or "put to sleep" means to be killed or euthanised when we are referring to animals

Answer

Adding a bit more -

In the US, in certain types of dog rescue centers, they can't keep dogs indefinitely.  After a dog has lived in the center for a certain period of time, it is euthanized.  The dog in the story had been in the rescue center for a long time.

Lucky family for finding their dog just in time!

Answer

The dog was about to be euthanized. The dog is lucky!