Rust: What is the difference between ? and .unwrap()?

1// ? — safe error propagation
2fn read_file() -> Result<String, io::Error> {
3    let content = std::fs::read_to_string("file.txt")?;
4    Ok(content)
5}
6
7// .unwrap() — panics on error
8fn read_file_unsafe() -> String {
9    std::fs::read_to_string("file.txt").unwrap()
10}