High kill shelters would refer to animal shelters that have a tendency to euthanize animals at a rapid rate.
Animals that are "unwanted" or abandoned at high kill shelters have less than a one in five chance of making it out alive. Contrary to popular belief, the shelter does not discriminate against an animal when killing them. Puppies and kittens along with purebred animals are euthanized every day; neo-natal kittens and puppies (bottle feeders) are often killed ON INTAKE!!!!
These facilities kill the animals within ONE TO FIVE DAYS of arrival - some will kill them ON INTAKE. For example, if one takes a "stray" animal to any County Animal Services, it is given a FIVE DAY HOLD by law (unless severely injured or ill - in that case, the animal is often killed within a short time of arrival.) If an OWNER takes their animal to County Animal Services, (most commonly referred to as "owner surrender") it may only get ONE day to be adopted, but the facility is NOT REQUIRED BY LAW TO HOLD THEM. Needless to say, if the owner changes their mind, and wants to reclaim their animal, there is a strong chance they will NOT be able to do so!
Many of the facilities maintain either a full capacity, or in some places, OVER FULL, so it is important to know that if an animal arrives that is not in "good" condition, or appears "feral", that animal has virtually NO CHANCE of making it out alive if the facility is at capacity. The "high kill shelters" kill animals - sometimes daily - to "make space" for tomorrow's arrivals. Some of these facilities receive up to 100 animals PER DAY!
These facilities sometimes have bouts of illness that affect many animals at once - one dog with parvo or one kitty with Panleukopenia can cause the facility to kill off EVERY ANIMAL IN THAT BUILDING at one time, though these are treatable diseases, they are potentially deadly and very contagious in the wrong environment. Often, by the time symptoms appear in an animal, it is too late to keep others from catching it, (though many will not) so the facilities use killing as a measure to control disease as well. It is very sad when this happens, but for some reason (most likely the usual "budgetary issues"), it's the only solution the facilities have come up with to "rid the facility" of the diseases when they occur in numerous animals at once.
Needless to say, it is important to make sure your beloved fur-baby NEVER ends up at one of these high kill facilities!
SPAYING AND NEUTERING IS THE ONLY LONG TERM SOLUTION TO STOP THE KILLING AND SOLVE THE "OVERPOPULATION" PROBLEM WE CURRENTLY HAVE!!