The youtube poster asked for a “cat expert” to explain why the black cat was meowing like that at the orange cat.
And…I wouldn’t call myself an “expert”, but I’ve seen and heard enough of that sort of thing to bet that the orange cat is an “interloper” on the black cat’s territory and the black cat is trying to intimidate him off. He’s getting all up in the other cat’s face and slinking around with his tail all bristly but NOT physically attacking. Which makes me think he probably doesn’t know the other cat very well and isn’t sure of his capabilities.
[Cats will actually usually go out of their way NOT to fight, even male cats, and even if it eventually comes to blows, there tends to be a ton of posturing and mutual attempts to make a big show of body language and noises first. Which is pretty much what this whole video is.]
Oh and in the beginning half, the way the orange cat is acting reminds me of how Shadow was, even as a kitten, when he first arrived (after 3 weeks of separation from siblings Cora and Brodie, who I brought home first after the first trapping day). Cora is good friends with him now but as babies they had this whole rivalry thing going on, and Cora would growl and do puffy-tail and glare if Shadow got up on the bed where she was, etc. And Shadow would just sit there looking like if he were human he’d be going, “I’m ignoring youuuuu….”. Eventually the orange cat in the above video does slink off, but he stood his ground for quite a while before that.