"Dependence" on something means that you need it. "Reducing" something means making the number or amount lower.

In the U.S., one of the main reasons that people discuss for wanting to develop green energy sources like solar energy and wind energy is to "reduce the country's dependence on foreign oil". This means that the country won't need to buy oil from other countries.

Reducing the country's dependence on foreign oil makes the country more independent because other countries won't be able to control the amount or cost of energy.