C++: What is RAII?

1class File {
2private:
3    std::fstream file;
4
5public:
6    File(const std::string& path) {
7        file.open(path);  // Acquire
8    }
9
10    ~File() {
11        if (file.is_open()) {
12            file.close();  // Release
13        }
14    }
15};
16
17{
18    File f("data.txt");
19    // Use file
20}  // Automatically closed!