By this point in the game, you've encountered - and hopefully rescued - Navali. She's along the path leading up to the Prison, so she's difficult to miss. Once you've rescued her, you'll find her in town. (If you did not rescue her for whatever reason, take the Prison waypoint, leave the Prison just to its northwest, and find her in The Climb.)
Navali offers your character Prophecies at the cost of 1 Silver Coin currency item apiece. You can have up to seven of them at a time. Prophecies are consumed when triggered. When you obtain a new Prophecy from Navali, your Prophecy window will pop up and display the new prophecy.
If you want to get the most bang for your Silver Coins, you should not seek Prophecies until the end-game. However, if you want to use them now, you're not likely to seriously sabotage yourself.
Prophecies can do a whole lot of different things, too many to list in full detail here. But they come in a few broad groups:
- Prophecy Chains: These prophecies are indicated by a I, II, III, IV, etc. after their name. Once you've completed the first in the chain, you'll be eligible to get (though not guaranteed to get) the second when you seek a Prophecy. Most of these chains have a major reward at the end of them.
- Fated Unique Upgrades: These prophecies are worded as "You will kill <Boss> while holding <Item>". Killing the relevant boss with the named item in your inventory will upgrade the item into a new, and usually much more powerful, form. These prophecies (and the Unique items they create) can be very valuable in some cases.
- Tempests: Worded as "You will enter an area with a <type> Tempest". Tempests apply to a whole area, and create circles that give either beneficial or detrimental effects to both you and enemies depending on the type of Tempest chosen. They also increase the number of drops in the area they're present in.
- Unique Drops: Worded as "You will <track down> a powerful <monster type>, which will drop <something> when slain". If the language is "powerful", it refers to a Rare (yellow) rarity monster. If the language is "very powerful", it refers to a Unique (orange) rarity monster.
- Encounters: Worded as "You will encounter <something>". Most of these prophecies have no direct reward, but they add additional monsters which may have drops.
- Others: These are worded in various ways, but most of them are fairly self-explanatory. Some of these - like one that guarantees creation of a six-link item - can be very valuable, and you should be sure to read any prophecies you get to be sure you don't waste them.
Triggering & Sealing Prophecies
Prophecies are not manually triggered, and it is possible to accidentally trigger one! In particular, if you have a valuable prophecy triggered by certain conditions (like entering an area), make sure not to satisfy those conditions on a zone where you do not want the Prophecy to trigger.
In order to stop a prophecy from triggering, or to remove a prophecy from your prophecy pane, you can Seal it. Sealing a prophecy costs Silver Coins, and converts the prophecy into a tradeable item. Since a Sealed prophecy is no longer active, it won't trigger, so you can safely go about your business without worrying about triggering it.
Because Sealing costs Silver Coins, it's usually only worth doing on Prophecies with some significant value, or ones you don't expect to do for a while that you just want to remove from your Prophecy pane to make room for others.