GDP is gross domestic product, a measurement of the total value of goods and services produced by a country in a given year. Every t-shirt, car, Apple, candlestick, massage, medical treatment... Every thing people in a country buy or service people pay for. This measures roughly the economic output generated by that country over the course of time. It's one of many measurements used to reflect what is going on in an economy. More useful is GDP per capita, which is simply the GDP divided by the number if people living in the country. So a country with a high GDP but also a high population such as China would have a lower GDP/cap than the United States which has a higher GDP and a lower population.
Take all of the money every person, corporation and governmental body in the country spends. Add in the amount of exports the country makes, but subtract the amount of imports. That's the GDP. It's a measure of the size of the economy.