A rod licence is a statutory requirement (in other words it is required by law) and entitles the licencee to use a rod and line to fish. Rod licences are issued by the Environment Agency. They differ from permits (often referred to as tickets or day tickets) which give permission to fish a particular water or waters and are issued by fishing owners, angling clubs and so on.
Rod licences can be purchased from Post Offices in England and Wales, and using the online form via the Environment Agency web link below.
http://www.environment-agency.gov.uk/subjects/fish/?lang=_e