Cover

From Guild Wars Wiki

"Cover" is a term that can be applied to a hex, enchantment, or condition that is meant to be applied on top of another effect of the same type in order to prevent the original from being removed.

Since enchant, hex, and condition removal all remove the most recently applied effect, it is important in PvP (and some areas of PvE with heavy removal) to cover all strong effects with cheap, fast-casting cover effects.

For instance, if a Necromancer puts Depravity on an enemy Monk, another enemy who uses Inspired Hex on the monk will undo the necro's work. However, if the Necro (or another player) first puts Ether Phantom on top of Depravity, Ether will "cover" the hex, and be removed instead of the more important elite. In this specific example, Ether Phantom accomplishes goals similar to Depravity's, while still being a cheap, reusable cover hex.

The same principles apply to conditions and enchants. A crippled Warrior is not long hindered if there are no other conditions to prevent quick removal of crippling. Likewise an entire build based on an enchantment like Spirit's Strength requires cover enchants to ensure the build can function.