Mono-Type Pokemon are Pokemon that has a single Type. For example: Typhlosion is a Mono-Type Pokemon. It's base type is, Fire. So it gives a better boost to Fire-Based moves. Its STAB boost is greater than Dual-Type Pokemon because Mono-Type Pokemon are naturally powerful that way.

Now, a Dual-Type Pokemon, such as Infernape, is Fire/Fighting. It boosts Fire and Fighting moves but a weaker STAB boost than Mono-Type Pokemon. However, they can still deal inflict greater damage if used right.

Now let's move on to why Mono-Type Pokemon are better than Dual-Type Pokemon. You see, certain Pokemon, like Swampert, has only one weakness and that's Grass. But, Pokemon like Scyther has way too many weaknesses. Mono-Type Pokemon usually have 1 or 3 weaknesses and they usually have a better advantage than Dual-Type Pokemon.

So, if I were using Typhlosion now, its Eruption (if it has full HP) deals tremendous damage to my opponent. Escavalier would fall in defeat rather easily due to Typhlosion's Type Advantage and better STAB boost. And... Haxorus, is a wonderful Mono-Type Pokemon to have: it will boost Dragon Moves much better than Dragonite. However, they're both pretty powerful so... Anyways, you can use both Mono-Type and Dual-Type Pokemon to your advantage. It's a necessary thing to do, only if you play your part right.

By the way, STAB stands for "Same Type Attack Bonus." Mono-Type Pokemon gets *2.0 STAB Boost and Dual-Type gets *1.5 STAB Boost.

Remember that now.