Relics are a strong tier of custom item that can be obtained in various ways. An unopened Relic (base item Magma Cream) can be opened by right-clicking the item. It will give you one Relic tier item from the currently active Relic pool. Relic tier items are often regarded as stronger than Mythic Tier, and weaker than Arcana tier. Relics are made by the Items Team, via community suggestions, or even special suggestion events.
There are a number of ways to obtain Relics, including voting for the server, opening crates, collecting your daily reward, buying from the Shady Trader at $30,000 each, or crafting them with Relic Essence.