A suit in poker is the other characteristic that marks the card, besides its number or value in the ranking hierarchy. The card can be suited in clubs, diamonds, spades, or hearts.
Suits are only relevant for making hand combinations like a flush, straight flush, and royal flush.
Although suits usually don’t influence the strength of a poker hand (i.e. an Ace-high flush in spades is equal in strength as an Ace-high flush in clubs), they’re still ranked: clubs (being the lowest suit), diamonds, hearts, and spades.
These rankings can come into play when determining the dealer position. If two players are dealt the same high card, the one with the stronger suit will be the button.