Evenly spaced firing order (also called even firing order or even firing interval) means that the angle between each firing is equal.
Engines that are even-firing will sound more smooth and steady, while engines that are odd, or uneven firing will have a burble or a throaty, growling sound in the engine note, and, depending on the crankshaft design, will often have more vibrations due to the unevenness of power delivery.