Liberty High guidance counselor Mr. Porter is on the receiving end of some serious threats in the trailer, which tell him to "know his place." The photos floating through the air also seem to show that the windows have been smashed in his office, with the threatening note being sprayed over a bulletin board of flyers. As a refresher, Mr. Porter is on Hannah's final tape because of his inability to give her the help or attention that she so desperately needs when she goes to him for a counseling session after being raped by Bryce. She records their entire session, which includes the shameful moment when Mr. Porter tells her she needs to just "move on" from being sexually assaulted.
As for what "know your place" means specifically, we have to assume that Mr. Porter becomes integral to the court case. Given how the school could be on the hook for the blame against Hannah — for creating/allowing Hannah to continue being exposed to an unsafe environment — it sounds like the threat is coming from someone above him.