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ELI5: What exactly is GDP in economics and how does it affect nations and their economies?

I'm really trying to grasp this concept but I can't seem to wrap my head around it. I have the most basic knowledge of economics and I hear GDP being referenced often though have no clue what it means or does, and I can't really find any information that describes it simply. ELI5!
gpt
GDP stands for "Gross Domestic Product". In short, it's the total value of everything produced in that country over a given period of time (typically a year). 

In an ideal world, this value would be perfectly visible. We could see a car roll off the assembly line, and a little video game-style score number would pop up above it to tell us exactly how valuable that car is. In practice, we can't see some abstract idea of value, so instead we use prices. If somebody went and paid $25k for that car, that's how valuable it was. Calculating GDP is just a very careful process of summing up all the purchases in an economy while being careful to avoid double-counting (the car factory bought tires to put on the car, but the final customer also bought those tires, so we shouldn't count both purchases). 

The higher a country's GDP, the more "stuff" it makes. Big countries will make more stuff than small countries just by construction. If you divide all that stuff by the number of people, you get "GDP per capita". Under the (false) assumption that all the stuff we make is actually divided equally among all citizens, each person can expect to get goods worth the GDP per capita.\* Therefore, it's generally better for GDP per capita to be high and growing rather than low and shrinking. 

Because we can't see value perfectly, our technique of calculating GDP means that lots of activities of questionable value are included in GDP, and lots of activities that contribute real value are not. If you crashed your car and then bought a replacement, that contributes to GDP. Meanwhile, if you grew your own vegetables and prepared a meal at home, that does not contribute to GDP because you never made any purchases we can observe. On the other hand, if you bought a meal in a restaurant, that does contribute to GDP. 

\*In reality, people need to pay for the stuff they receive. They typically pay with money they earned from working, that is, making stuff. To a rough approximation, if you're very productive and make a lot of stuff, you will have a high wage, which will entitle you to consume more stuff.