GDP/C is the Gross Domestic Product per capita (head). This value can best be decribed as a kind of "income" per person and a measurement of production. Higher GDP/c can be caused by low unemployment, as employers compete for the workforce, and by an increase in production. Tax income is primarily affected by the GDP/c. It is affected by taxes, domestic prices,unemployment and social spendings. Higher GDP causes higher demand for services which costs more.
GDP/c will also affect the demand of your people for commodities they consume such as food and consumer goods.
GDP/c also has a direct effect on Domestic Approval Rating, in regions where DAR applies.
Note however that while GDP/c is an indicator for the personall welfare of the people it also might be an indicator (especially together with low unemployment) for quickly rising production prices (the people get high wages) which might soon send the economy into a recession.
Higher GDP/c means also higher production costs and this makes you much less competitive on World Market (your products cost more).