Some programs --- specifically, solutions of most tasks prior to the December 2020 contest --- will read their input from a file named in the problem statement (e.g., `cow.in'). If you are using file input, do not specify a complete path name in your `open' statement, just `cow.in'. Note that filenames are case-sensitive. If the problem says `cow.in' then you must use `cow.in' since `CoW.In' and 'COW.IN' will not work. Some programs write their output to a file named in the problem statement (e.g., `cow.out'); do not specify a path name in your `open' statement, just `cow.out'. Like the input filenames, output filenames are case-sensitive. If a problem uses file input / output, then output written to stdout or stderr will not be graded. Note that as of the December 2020 contest, input and output switched from file-based to terminal-based, using standard input and standard output (e.g., cin and cout in C++). The input and output spec of a problem will specify which mode is being used.

这是什么意思，开了翻译没看懂，谁能解释一下？