Cats seem to have a language all their own, just like other animals. That is certainly being displayed her as a mother cat calls out to her kittens to join her. The sound that mother cat gives is quite different then the usual meow we would expect from a cat her size but this is used as a way to get the kittens attention.
As any good mother would this mother cat wants to know what her kittens are up to. They have wondered off no doubt to get into mischief all their own. Instead of going off to search for each one individually the mother cat start to make a loud sound. This sound could be mistaken for her being in agony by a human, but to a kitten this sound means she is looking for them!
Shortly after she starts to wail the kittens start to assemble around her wanting to know what she wanted. So, for a cat the loud wailing sound is the equivalent of a parent calling a kid to come home. It seems to be just as effective if not more so! LOL!
Anyway, watch this mother cat gather her kittens around just by calling out to them in a way that only a cat could understand, then comment below on what you thought of the video. https://youtu.be/oaD-o_9Wgs0