Here you will find theoretical material about buffers.
According to Wikipedia, a buffer (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is added to it and thus it is used to prevent changes in the pH of a solution. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. Many life forms thrive only in a relatively small pH range so they utilize a buffer solution to maintain a constant pH. One example of a buffer solution found in nature is blood.
So let’s start:
Here is another great text:
If you have ever get lost with buffers, remember this flowchart:
HERE you can find some good study material, too.
Big thanks to https://www.applichem.com/fileadmin/Broschueren/BioBuffer.pdf and www2.sunysuffolk.edu/joshiv/handouts/PDF/Buffers_Handout.pdf.