This is 5 cards that are all of the same suit (such as Clubs or Spades) and in any order of rank. When the cards are in ascending order then the player actually has a Straight Flush and this is worth more.
In a showdown where 2 players both have a Flush, they compare their highest value cards to find the winner. If these cards tie, they go down in order of descending value to find a winner.
The ‘nut flush’ is the best possible flush you can have that is not in ascending order. Generally, the ace-high flush is the nut flush, but if the ace is on the board, then the community cards can come into play.