An uncodified constitution is one that can be updated through the addition of precedents and customs, and by establishing best practice. The constitution determines the bottom line, but the interpretation can be made through precedent and by creating procedures that are always followed.
The U.S. Constitution is an uncodified constitution, as it incorporates precedent cases within existing laws.
The uncodified method allows for the constitution to be updated as it needs to be through the progress of time as well as changes in technology and other behaviors. Examples include the addition of gay marriage to laws regarding anti-discriminatory laws, as well as updating harassment laws to include online harassment.