Privacy is the notion that the government will not intrude upon actions that you intend to be private. For instance, intercepting someones mail violates their privacy, as it is expected that only the recipient of the mail will read it.
Every infringement upon privacy can therefore potentially result in a infringement of liberty. For example, a wiretap could lead to an arrest.
People have a right to privacy, that is, they have a right to expect that the government will do as little as necessary to intrude upon their rights. This right is expressly provided for in the bill of rights. These rules govern due process. This is why the police must obtain a warrant to do a search