A country's gross domestic product is the value of everything that was produced and sold by that country in a year. The word gross in this phrase doesn't mean "yucky" — it means "total."
Gross domestic product, or GDP, is one way to judge how a country is doing economically (domestic means "relating to or produced in a particular country"). GDP includes everything produced inside the country, including services and goods, and is a survey of the entire economy that can be compared to previous years (or shorter time periods). By comparing different years' gross domestic products, economists can see if the economy is growing.