Cats, unlike dogs, are obligate carnivores. Meat is their most important source of energy.
During digestion, the protein in the meat is broken down into its individual components, the amino acids. Intestinal bacteria, which count among the natural gut flora (= intestinal microbiome), break down the amino acids into precursors of uraemic toxins. These precursors are then resorbed and pass via the liver to the kidneys. The uraemic toxins are poisonous degradation products of the protein metabolism which the cat’s body is unable to metabolise so that it has to excrete them via the kidneys.
In healthy cats, this is not a problem, since the kidneys continuously excrete uraemic toxins. If the kidney function is impaired, however, e.g. in older cats, uraemic toxins – indoxyl sulphate in particular, here – remain and accumulate in the body.
That is when their harmful effect can come into play.