In the olden days, as my kids used to say before they had kids of their own, it wasn't rare for a ham to build a one-tube rig and put it on the air just by coupling the output to the antenna. The amount of coupling and the degree of mismatch could seriously detune the rig, so most of us built buffer stages which serve to isolate the oscillator (the one tube rig) from the antenna. Changes in the antenna thereby had much less effect on the oscillator, resulting in much greater stability and better output quality.
Then came the pi-network outputs, multi-807 finals (later 6146s) and so forth.
So a "buffer" is a stage that isolates sensitive stages from unwanted influences.