Redwood City is a “charter city” meaning that the legal authority for the City's actions originates with a legal document called a City Charter, rather than from the laws of the State of California (although the Charter must generally be consistent with State laws, with only a few exceptions).
Charters are adopted by cities where special conditions create needs that can’t be adequately met by the general laws. A city can adopt a charter and tailor its organization and elective offices to provide for unique local conditions and needs. A charter can only be adopted and/or changed by a majority vote of city residents -- not by a vote of the city council.
Citizens can establish the terms and number of council members and impose other limitations upon their city council through a charter provision. Among other things, the City Charter gives the City Council the authority to adopt codes and ordinances in order to regulate certain issues within the City.