What does 'buffering' mean?

Question

I read the lecture 4 of CS162 (UC Berkeley: https://inst.eecs.berkeley.edu/~cs162/sp17/static/lectures/4.pdf), but I got little bit confused about the meaning of the keyword "buffered" they used when mentioning the following informations:

Kernel buffered reads

Kernel buffered writes

Data buffered automatically by C-library functions

What does each of 'buffering' and 'buffered' mean?