In simple words opportunity means possibility of success. It can also be described as something that improves the likelihood that a goal can be attained. Nonetheless there are many other many other factors that can determine the outcome such as one's will, interests, motivation, knowledge and intelligence, for example.
Liberalism is a theory of individualism put into a political context. Guaranteeing individual rights and liberties are crucial elements of this ideology. Liberalism strives to secure "equal opportunity" not in a social democratic, socialist or communist way, ie, through focusing on the achievement of material equality via redistribution of wealth by the government. Instead the aim is to create a system that allows individuals to succeed through use of their own abilities and resources. It focuses on personal achievement and accomplishment.
Liberalism stands for political equality (guaranteeing citizens rights and liberties) but does not intervene into the private sphere (non-interventionism). It considers private property to be of crucial importance in the creation of economic and hence social well-being. Opportunity for liberals means an equal chance for every citizen to succeed in life and accomplish his/her dream in a competitive environment and with due regard for the rights of the individual