The "buffer" values are the ones previously written to the destination buffer. For example, for a depth test, this would be a value in the depth buffer. That value was either put there when you cleared the depth buffer at the top of the frame, or by writing the result of a previous fragment that itself passed the depth test.

The "buffer" values are the ones previously written to the destination buffer. For example, for a depth test, this would be a value in the depth buffer. 

That value was either put there when you cleared the depth buffer at the start of the frame, or by writing the result of a previous fragment that itself passed the depth test.

This is similar for blending tests (which operate on the color buffer) and stencil tests (which operate on the stencil buffer). You put initial default values there at the start of the frame (most of the time), and potentially modify what is in them after each fragment operation.