What do these things mean: 0x01, 071 in C language?

Question

Number systems

What is meant by such a notation, when is it useful to use it?

Answer 1

"0x" means that 01 is not in decimal, but in hexadecimal number system.
It's mostly useful when you're getting this value from documentation, where it's more common to use hexadecimal than 10. This is usually the case in the documentation: all constants are hexadecimal.