I am not a lawyer, but basically it means he is innocent until proven otherwise and he has the right to not testify in a way that might incriminate himself and he is exercising it.

There is the sort of popular view that if you do this, it must mean you are guilty and by testifying you would actually be proven so. The problem with that is that you might actually be innocent but not be able to give evidence in a way that does not make you look guilty.

Any lawyers here willing to elaborate on how to better view this from a social point of view?