A copy/paste out of the 737 FCOM:
The takeoff configuration warning is armed when the airplane is on the ground and either or both forward thrust levers are advanced for takeoff. An intermittent warning horn sounds if:
- trailing edge flaps are not in the flaps 1 through 25 takeoff range, or
- trailing edge flaps are in a skew or asymmetry condition, or have uncommanded motion, or
- leading edge devices are not configured for takeoff or have uncommanded motion, or
- speed brake lever is not in the DOWN position, or
- spoiler control valve is open providing pressurized hydraulic fluid to the ground spoiler interlock valve, or
- parking brake is set, or
- stabilizer trim not set in the takeoff range.
Hope it helps