What does it mean to be on a gold standard? When currencies are on a gold standard, why are discoveries of gold more likely after a period of deflation?

When a country is on a gold standard, this means that an ounce of gold can always be exchanged for a fixed quantity of currency. For many years before the 1930s, an ounce of gold was always worth $35.00. 
Imagine that the prices of goods and services fall in dollar terms. Then, for a country on a gold standard, an ounce of gold will be able to buy more goods and services. This makes searching for gold more attractive, and so more effort is put in to finding it.