The "straight flush" is a quite unusual rank. To have a straight flush, you must hold 5 cards that qualify both as either an even or odd straight and as a flush. In other words you must hold 5 cards with consecutive even or odd values and identical suits. (Please see the sections on "Odd and Even Straights" and "Flushes" for more precise definitions.) Below is an example of a "Straight Flush":
The cards are all hearts (same suit), and they have consecutive odd values (3, 5, 7, 9, J).