Bith is the Middle English spelling of the Old English biþ; it is the third-person present singular of bēon, the verb we have mutilated into the modern 'to be'.
Today we would say "he is", "she is", or "it is"; in Middle English it would be "he bith", "sche bith", or "hit bith".
This evolved into beeth and bes before the great English verb die-off, when everything went all is/am/are.