Having a gold resource means knowing with some confidence that an amount of gold at a known grade exists in the ground. Without a gold resource, a company cannot begin the first steps of planning a mine.
The certainty that a given amount of gold really exists in the ground will depend on how much data is available and the complexity of the gold deposit. A minimum amount of data and confidence is required under the various reporting jurisdictions in order for a company to make claim that they have a resource. The most commonly used are the JORC Code and NI-143 as these are the reporting standards required by the Australian and Canadian Stock Exchanges respectively.
While the resource reporting rules vary in their detail (summarized here), their aim is reported resources not mislead investors, either through poor technical work, or straight out fraud.