Dangerous is the risk of death in a certain place, minigame or dungeon. In the case of minigames, they are classified as Dangerous or Safe, depending on the risk of death, where items are lost. Examples of dangerous minigames are the Mage Arena, Barrows or the Crucible. Examples of safe minigames are Stealing Creation, and Fishing Trawler in which the only way you could die here is be poisoned before entering the arena.
In the case of areas or Dungeons, this classification is much more open to interpretation, but is normally considered safe when the monsters do not pose a threat to anyone above level 20-30, and below that level a player can easily survive with food.
One of the most dangerous areas is the God Wars Dungeon, classified by Jagex as the most dangerous place in RuneScape. It is very easy for a player to die, due to the high level monsters that populate the dungeon.
Players are often pre-warned before entering a dangerous dungeon or area (such as the wilderness), and are required to click to continue. These warnings can be toggled off at the Doomsayer in Lumbridge. One example is when you enter another floor in the Stronghold of Security, in Barbarian Village.
However, dangerous activities are often the activities that a player can profit from the most, a very good example is God Wars Dungeon. Plenty of people have made millions from godsword shards, hilts, as well as other assorted items unique to the God Wars Dungeon.
If a player is unsure about the danger level of a certain minigame, area or dungeon they are advised to ask to fellow players, in-game or in the forums, check on RuneScape Wiki, or use the in-game dungeon map where it shows what monsters lie ahead, their levels and their numbers. Bring food to dangerous places to live a bit longer. Remember that even if the combat levels of monster are higher, you might still be able to run past/fight them. With the run energy update, it is easier to stop at a safe spot to rest for a while, and run past the dangerous monsters, to reach a deep point in the dungeon/area.
High-level players would trick new players to go into dangerous places, like the old wilderness, in which the new players would die. Then the high-level players would take the dropped items. This is reportable under Honour.
Boss scenarios, such as those out of quests or tough enemies, are VERY dangerous if you do not bring the correct equipment. However, this does not apply to the Tormented demon boss fight in While Guthix Sleeps, as you have near infinite life points. A dangerous boss fight might be the King Black Dragon if you do not bring an Anti-dragon shield, don't have the prayer level for Protect from Mage, or an Anti-Fire potion.
Places that are also considered dangerous are dungeons in which you need a light source. In this case, if your light goes out, you will get two messages in your text box. If you don't light your light fast enough, you'll continually be hit for 1-10 until you die, relight the light source, or teleport. If you do die in these situations, it is recommended you have a spare light source in order to retrieve your items. Another dangerous place is the Clan Wars red portal. If you die in the red portal, you WILL lose all your items.