The clutch is a third pedal to the left of the brake which lets you disengage the engine and transmission so you can change the gear then let the pedal out, engaging the new gear.

With a clutch, the brake pedal is usually really narrow. So when you get into an automatic instincts will tell you to press the clutch and change gears but that pedal doesn’t exist and the wide brake pedal is there instead. Instead of changing gears, you slam the brake.