What is a "Bounder" ??? Pls read on:

[Voronwe]

What the Bounders were is explained in the Prologue. They were basically Shirriffs whose job was to control the entry of 'outsiders' to the shire at border crossings:

A rather larger body, varying at need, was employed to 'beat the bounds', and to see that Outsiders of any kind, great or small, did not make themselves a nuisance.
At the time when this story begins the Bounders, as they were called, had been greatly increased.