In which way is what one calls "self censorship" different than "freely deciding what to say"?

I can't imagine how a concept as "censorship" can be applied to oneself.

If you can freely decide what to say, that means you can freely decide what not to say. It seem to me that te definition of "self censoship", because of that "self", simply overlaps with the definition of "freedom of speech".

Certainly, the reasons behind one's choice of what to say and what not may be very different. This is however something which has no relationship with "censorship."