A Flush is a hand where all five cards are from the same suit. The relative ranking of flush hands depends on the ranks of their individual cards.
Flush hands are compared starting with a hand's highest ranked card. If hand A's highest ranked card is higher than hand B's, then hand A is ranked higher. If hands share the same highest card rank, the comparison of card ranks moves to each hand's second highest ranked card. This card by card comparison is repeated until a hand has a card with a higher relative rank, or all five cards are of equal rank. In the case where all five cards are of equal rank, the hands are equal.
The highest possible flush is an Ace-high flush. The lowest possible flush is a Six-high flush. A Five-high flush is not possible since it would be a straight flush.