what this fallbacktoplaygroud:true does here

Pretty much what it says: https://github.com/ory/integrations/blob/67937c781e1587b3440147478763dc42ddabf75a/src/next-edge/index.ts#L108-L111 It is a convenience flag, so your setup works against the public playground of Ory Cloud if you did not provide configuration for your own project.