It is the practice of making tweets invisible to the general public in an attempt to censor a particular user. That is the truth. A more politically correct answer is that it is the technique of blocking a user that so that they don’t know that they are being blocked. It is pure deception.
Originally, the concept was intended to reduce spam or undesirable posts from a user that was considered a problem. However, there is a potential that this practice can be abused and used for nefarious purposes. Who judges whether a user is a problem or not and what motives are behind that decision?
I really became interested in this because after the topic came up on twitter. A well known cartoonist named Scott Adams wrote this blog post about shadow banning.