This is a tool to help hide your messages in plain sight, especially from people who do routine searches of a given term on a website like Tumblr or Twitter.
Simply put, it swaps out certain characters with visually identical, but differently-numbered characters. These numbers are what an algorithm uses to find a given term. For example, the term "Cat" is read by a search algorithm as "67, 97, 116", and the algorithm then checks for that sequence in your post. By swapping out "C" and "a" with the visually similar Cyrillic characters "С" (1057) and "a" (1072), your post reads "1057, 1072, 116", while the search term is still "67, 97, 116".The search algorithm won't find your post, but it still reads "Cat" to you and your followers.
That's the point!