By devaluing the Bolivar, the president of Venezuela has artificially decreased the value of the national currency.
What this does it make it more expensive to buy US dollars. In the last 2-3 years, there has been a flight of billion of US dollars out of Venezuela due to the economic disaster looming in the country.
By making it more expensive to buy US Dollars, the President of Venezuela is hoping to decrease the flight of money from the country and protect local reserves.
To what extent this actually works, we will have to see. In many developed countries or countries with a healthy economy, such a strategy might work. However, in the case of Venezuela, the economy is so bad, that even the devaluation of the Bolivar might not help solve much.