Danger level is a term used in-game to give guidance on the risks and rewards of different maps and areas on a map. Each tier corresponds to the types and number of creatures that can spawn there, as well as the rough quality of loot that can be found there. The higher the danger level, the more numerous items of higher rarities are.
Low danger
Striders, Rattlers
Moderate danger
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