What is the difference between read+write and read+write+truncate?

This is an easy question, but I couldn't find an explanation anywhere

I'm using fopen in C++, and I have the option to either use

"r+b" (Read and Write)

Or

w+b" (Read Write and Truncate)

What is the meaning of Truncate? What is the difference between "r+b" and "w+b"? I understand the mathematical meaning of truncate is removing precision after a certain number of decimal points, but I don't understand the exact meaning in the C++ context.

Thanks,