In analytical tools, such as Google Analytics, goal is a specific milestone the visitor is supposed to reach that is tracked and measured. Defining a goal in analytical tool is used to track and report the number of times visitors have fulfilled a particular goal (taken some action or reached a certain page on the website).
Most frequently used goals are destination goals, used to measure conversion rate. Destination goal is defined through designating a web page that will be a destination. Once goal is defined, analytical tool will start tracking the fulfillment of the specified goal.
It is possible to define a sequence of pages that visitor must go through before reaching the destination page, creating a funnel. It is most widely used to track micro conversions and estimate the engagement of the visitors with the site.