Verb
let know (third-person singular simple present lets know, present participle letting know, simple past and past participle let know)
(ditransitive) To inform (someone) (of something).
I sent an email to let John know what we found.
Let me know when you are arriving.
"I'm not sure which flight I'll be taking." / "Well, as soon as you find out, let me know."
If anyone has seen him, let us know of his whereabouts.