Many items in Path of Exile have a drop level. This dictates the minimum required item level for an item to drop.
This is not to be confused with the item level (a property of an actual item), nor the level requirement on a piece of equipment (which is determined by a combination of the item’s base type and implicit and explicit modifiers). These are both separate statistics.
Drop levels are not visible in-game, and can only be found by datamining.
Normal, magic, and rare equipment basetypes usually have a drop level equal to its level requirement as a normal item. Unique items may have different drop levels than their basetype, and can vary even between uniques with the same basetype.
A well-known example is Skyforth, which has a drop level of 84. As such, under normal circumstances, it can only be found from Rare and Unique monsters in T15 Maps (area level 82, +2 item level from monster rarity), and from Magic or higher monsters in T16 Maps (area level 83,+1/+2 item level from monster rarity).
Items without a specified drop level will typically use the level requirement of the item.
Divination cards have specific drop levels starting from 23; these can affect which areas they can be found in. Note that some divination cards also have an upper cap to drop level. For example, The Trial has a drop level cap of 77 – it cannot drop naturally from an area level of 78+ (T11+ maps). Note that sources of random divination cards (e.g. Eldritch Altars) ignore these limits.
Maps follow a slightly different rule from other items; maps can typically only be found at the same tier as the current area level. Magic, Rare, and Unique can drop maps up to 1 tier higher than the current map. Map bosses can drop maps up to 2 tiers higher than the current map. Tier 1 maps can be found in the acts from areas of level 60+.