Search is a special Action that may be performed in exploration scenarios. PCs can attempt to Search for anything Hidden within their occupied region. Common examples of Hidden objects or features would include treasure, Traps, some Creatures, exits, and devices such as switches. Each Craft provides a PC with their base Search roll.
Search rolls are opposed by Hidden rolls from each entity or other Hidden object or feature within a region. The DM makes all Hidden rolls in secret. Ties for this opposed roll are in favor of the Search. If the PC fails the opposed roll, they are simply told that they find nothing; otherwise they are informed of each thing found. If there is nothing Hidden within a region, the DM still makes a (fake) secret roll; the player is informed that they found nothing regardless of the result of their Search roll.
For more information, see Player's Guide Chapter 5: Exploration.