First of all, MOLLE (pronounced 'molly') stands for Modular Lightweight Load-carrying Equipment - so a MOLLE pouch is essentially any pouch that you can either standalone carry or attach to a belt of larger pack, which can be used to carry gear. Where a MOLLE pouch becomes valuable is if you want to carry it on your belt for easy access. For instance carrying items like maps, compass, fire starter or even ammunition, you'll want to make sure that you can access them easily while on the move. If they were in your backpack, it may take a little while to retrieve them quickly. So a pouch becomes super functional in that you can carry a small number of necessities within easy access.
MOLLE pouches are usually attached via a series of nylon hoops which you'll find on a lot of military backpacks. On the backpack you'll usually find a row or rows of heavy duty nylon fabric, which are called PALS (which stands for pouch attachment ladder system). There will be matching systems on a MOLLE pouch, which you can then match up with the backpack and attach through accordingly. Alternatively, if you haven't got PALS on your pack, you can always use some cord to attach your MOLLE pack to your belt or pack.
When it comes to choosing the best molle pouch for you, it comes down to your specific needs and making sure you're getting the best quality of your money. A good military MOLLE pouch is far superior to any standard pouch that you may get in a camping store for a couple of reasons. First of all, MOLLE usually means that it's designed to be easily attached to your existing backpack - which also means that it will be designed with durability and hard usage in mind. Make sure that you go for a MOLLE pouch that's made from durable nylon material to ensure that it doesn't end up wearing through it's hoops after long time usage. There would be nothing worse than hiking for a day only to discover that your pouch has been lost along the way due to the material being worn through. You'll also want to consider the size of your pouch. If it's going to be used for something simple like a first aid kit, then you won't need anything too large to get the job done - but likewise, make sure you don't got for something too small that's not going to have enough space to carry everything that you need.
MOLLE pouches in the military are usually used to carry additional equipment such as ammo, gas masks, hand to hand combat weapons, grenades or flares. But in a civilian or survival scenario, they can be used in any number of ways to help with your situation. For instance, using a MOLLE pouch to host your first aid kit is a great way of keeping your health essentials within reach. Likewise, if you're on a hunting expedition and need to have rounds of ammunition close bay, this again is a great utility. Outside of what you could typically expect, a lot of MOLLE pouches are customised to the individual. Anything from being used as a pistol holster to a snack pouch - there's a million different ways to use a MOLLE pouch.