The "straight flush" is a quite unusual rank. To have a straight flush, you must hold 5 cards that qualify both as a straight and as a flush. In other words you must hold 5 cards with consecutive values and identical suits. (Please see the sections on "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 values (the Ace being treated as the value 10).