Pronounced: hēi gǎi kǔ jiào
Literally: dark reform bitter education
Means: “The labour camp is dark and reeducation through labour is bitter.”
Related phrases include:
(Reform through labour)
(1. shorthand for 劳动改造; 2. a prison camp)
(Reeducation through labour)
(shorthand for 劳动教养)
You can see/hear some of these terms, with English subtitles, between 7:10 and 8:47 of this Al-Jazeera investigative report.
The sign says:
Who are you
What is this place
Why have you come here
You can find the answers to those questions in the video linked above.