The most important element in the making of good wine is the quality of the grapes that are used. The virtues of the wine are hidden in the grape and if they are not there, there is no way to invent them. A winemaker who is intimately involved with the vineyard may have an advantage in knowing the quality and the qualities that he is dealing with in the wines in the cellar. The term 'estate bottled' means just that. The people (person) that are growing the grapes are the same people that are making the wine. This, of course, does not guarantee that an estate bottled wine will actually be better, but it offers an opportunity to enhance the quality of the wine with an enhanced knowledge of the grapes that make the wine.
Unlike many other terms that you might find on a wine bottle, terms like 'reserve', 'special reserve', 'vintner's choice',
etc., which may be used at the marketing department's discretion and have no actual legal definition, the term estate bottled is defined by the government and cannot be used unless the grapes and the winemaking are managed by the same entity.