A close period is a period, usually of two months, prior to the release of results during which directors and other insiders are not allowed to trade in a company's shares.
It is also usual for companies to refrain from making other price sensitive announcements (e.g. trading statements) during the close period. This is not required, but it is usually regarded as good practice.
Many companies issue a trading statement immediately before the start of the close period so that investors have information that is as up to date as possible during the close period.
It is always possible for a company to be overtaken by events and put in a position that makes it necessary to make an announcement during the close period.