In magic we use symbols frequently: scrawled on a candle, a piece of paper, or even in dirt. The symbol X is a particularly versatile symbol that belongs in anyone’s bag of tricks.
I believe in general that a symbol, like a word, should have a clear definition, that it should mean something, but X is often the symbol for the unknown. In algebra, for example, it is used as the symbol for the unknown quantity and does not necessarily have a precise definition, which is not the same thing as saying it is undefined. It may be a real number, for instance, or a whole number, or a prime number less than 100. Likewise X in a magical equation can stand for a specific unknown – perhaps a manuscript that is sought whose title is unknown. In detective fiction, X often stands for an unknown person, either the person who commits the crime or some other enactor of an anonymous deed. You would not be using X in your magic to summon a criminal (at least I hope not), but X could stand for an unknown donor or other helper.
The X shaped rune gyfu means “A sign of hospitality and friendship, of joy and celebration.”* It may represent an offering to the gods or some other kind of gift. This interpretation of gyfu relates to another important intimation of the symbol X: as a mark on a map indicating the location of buried treasure. X can mean treasure or it can refer to the place that is sought, be it literal or metaphorical.
*D. Jason Cooper, Using the Runes. (Wellingborough, UK: The Aquarian Press), 1986.