Definition: In statistics, an intervening variable is one that occurs between the independent and dependent variables. It is caused by the independent variable and is itself a cause of the dependent variable.
A higher education (independent variable) typically leads to a higher income (dependent variable). Occupation is an intervening variable here between education and income because it is causally affected by education and itself affects income. In other words, more schooling tends to mean a better job, which in turn tends to bring a higher income.