Equality is a simple measurement of the distribution of wealth. The higher the equality, the less it means to be 'wealthy' or 'poor.' Equality is raised by progressive taxes (which target the wealthy more than the poor, as opposed to flat taxes which target everyone equally) and opportunities to the poor.
Equality is primarily only a concern for a nation's Socialists. However, even when leading a very non-socialist country, it's often a good idea to have at least some level of equality; high amounts of inequality are the main contributor to Class Warfare.