A Significator is a Tarot card that is chosen to represent the querent being read for (even if it's yourself). While it isn't a necessary part of a reading, sometimes it can be a good focus for the reader, especially if the person isn't present, such as an e-mail, phone, or video reading.
Some readers prefer to choose a Court card that either "looks" like the querent (such as their physical coloring or their outward demeanor, or their age) - if known: a King for a mature male; a Queen for a mature female a Knight for a young man; and a Page for a child (male or female).
When using a Court card as a Significator, the "coloring" of the people in that suit should also be a considering factor: Coins/Pentacles - fair skin w/ blonde hair (Scandinavian type); Cups - medium skin w/ light brown to red hair (European/American type); Swords - black skin w/ black hair (African/Aborigine type); Wands - dark skin w/ brown to dark brown hair (Middle Eastern/Asian type).
Others prefer to choose from the Major Arcana, calling on the energy of the querent and/or the underlying basis of their question. This is my personal preference, but I also rely on what I'm feeling from and about the client and what their question is, and so may choose a Court card instead.
Choosing from the Major Arcana, the "look" of the person isn't as important as the deeper, inner aspects of the querent are. Are they in a relationship, and have a question about their love life? Use the Lovers card. Are they "artsy" and creative and looking to see how best to use their gifts? Use the Magician card. Are they about to embark on a new path in life? The Fool would work here. And of course, the proverbial finance question? The Wheel of Fortune has always been a good one for me to use. Sometimes in this latter type of question I may use the Emperor for a male or the Empress if a female, and then hope that the Wheel of Fortune comes up in a beneficial position.
So whether you choose a Significator or not is strictly a personal choice, but at least here I've presented a few ideas on how to do so, if you decide to try it out on your next reading. It's all personal preference, and if you find you're not comfortable with it, don't worry about it. How you read is your "style" and it's what works for you. Don't ever let anyone tell you that you "have" to do it in any specific manner. Just relax, read, and enjoy!