Danger levels are a term used in-game to give guidance on the risks and rewards of different maps. They do not refer to specific threats or specific Loot, but are generalised ratings ranging from low, to moderate, to high.
Low Danger Level
Striders, Rattlers
Moderate Danger Level
Striders, Rattlers (same as T1, just more numerous)
High Danger
Striders, Heavy Striders, Rattlers, Mature Rattlers, Marauders
Very High Danger
Striders, Heavy Striders, Rattlers, Mature Rattlers, Marauders (same as High, emphasis on elite spawns)
Extreme Danger
Striders, Heavy Striders, Rattlers, Mature Rattlers, Marauders, Savage Marauders