A significator is a card chosen from the deck to represent the querent, whether it be you or someone else.
Most commonly a Court Card is chosen, but some people use a Major Arcana card and some just deal a card randomly from the deck.
Some spreads require a significator, some do not. Personally, if a significator is called for, I just deal a card from the deck into that position, rather than choosing a specific Court Card. Whatever card falls in that position represents the querent in relation to the question at hand.
I feel pulling a specific card from the deck to use as a significator removes that card and prevents it from being dealt into another position in the spread where it might be more informative.
Many readers feel a significator provides focus to the spread however, and choose them carefully, feeling that the deck will chose another card to convey the same message if it is called for. In this, as in most things, you have to chose what feels right for you.