One of the most important things that an artifact can do is give you access to parts of the color pie that are normally outside of your decks range. One of the most powerful examples of this is Nenvindyral's Disk, which was a common staple in necropotence decks because it was one of the only ways that a mano color black deck could destroy an enchantment. There are others though, zuran orb would let anyone gain life and there are several card drawing artifacts.
Veilstone Amulet is rare artifact from the future sight expansion set of magic the gathering. It costs three mana to put into play and its ability reads "Whenever you cast a spell, creatures you control can't be the targets of spells or abilities your opponents control this turn."
This is an ability that can be used in numerous ways and any color would be generally happy to be able to use it but some are more aided by it than others. This basically comes down to how easily those colors can already protect their creatures. Blue for example already has counter spells as well as a few creatures with similar abilities and so the amulet becomes a little less useful for it, while red has virtually no access to counter spells and its creatures are generally unprotected as they have neither regeneration as you might find in black, or shroud which you often find in green. They even have small toughness in many cases making them even more vulnerable.
In the best cases this card can be used as a counter spell. Simply wait until your opponent has cast a spell targeting one of your creatures and then use an instant from your hand. The spell then no longer has a valid target and it will fail to work fizzle and going to the graveyard. And since most red mages have spells that can be cast for some value at any point in the game you can make it very difficult on your opponent.
In limited this card is useful because most games come down to the creature battle and if the opponent is unable to destroy your creatures he is likely to lose. Where it loses value is in the ability to activate it. Instants are not as common as one might think and many are not that useful so you will find yourself unable to use this much of the time making it a mid range choice.