"Not great, not terrible."
- – Anatoly Dyatlov, Chernobyl (2019)
The maximum dosage of radiation that can be accurately measured by a dosimeter rated for 3.6 roentgen. Side effects may include instant second degree sunburns, spontaneous vomiting, and a brilliant laser-like beam piercing up into the sky due to the Cherenkov effect.
When off their homeworld, the Death Korps of Krieg need at least 3.6 roentgen in their daily diet to stave off lethargy. Their Munitorum issue dietary supplements also work well as hand warmers, and can be piled together to cook meals and recharge lasgun packs.
Sly Marbo's Catachan chili recipe is measured in roentgen instead of the customary scoville units.
An explanation for the confused
It's a meme from a miniseries about the Chernobyl disaster. There are plenty of explanations available from Google, but if you insist: the short version is that the dosimeters at the Chernobyl plant were only rated for measuring 3.6 roentgen (an obsolete unit of measuring radioactivity); the good dosimeters were kept under lock and key, and were thus unavailable in the initial hours of the disaster. 3.6 roentgen would be plenty bad, but not horrible--roughly speaking, depending on the radiation, it would equate a less then a 5% increased probability of dying of cancer. Later, more accurate measurements suggested 15,000 roentgen of radiation, meaning the initial estimates were off by a factor of 4000. Mixed with Dyatlov's otherwise insane behavior in the hours after the explosion, the entire internet made the man a meme in the aftermath of the 2019 miniseries, with the "3.6 Roentgen" line particularly being widely memed on.
 Why yes, the Soviet system was horribly dysfunctional, thanks for asking.
 For scale, that's a bit like a claim that something is a foot long, when it's actually 3/4 of a mile long; or a claim that something lasted a second when it actually lasted over an hour, or a claim that something lasted an hour when it actually lasted 5 months.