What is the Marshall-Lerner Condition?

-Fall in Value of Currency will only Reduce a Current Account Deficit if the Marshall Lerner Condition Holds -This states that for a Fall in Value of Currency to lead to Improvement in Balance of Payments, the Price Elasticity of Demand for Imports + Price Elasticity of Demand for Exports must be Greater than 1 PED (for Imports) + PED (for Exports) > 1