It is the exposure of plants to cold or low temperature to enhance flowering.
Some plants have to have a number of hours of cold weather or they will not flower in spring. Many spring bulb flowers are this way. Winter wheat is a crop plant that needs winter cold to be able to make the wheat bloom.
According to the following websites, under proper conditions (soil, sun, vernalisation etc) the olive tree should begin bearing fruit when it is 5 years old. http://aggie-horticulture.tamu.edu/extension/fruit/olive/olive.htmlions http://www.eastofedenplants.co.uk/olive_guide.htm