They can be used to store runes as well as act as slots for inventory so you can carry more. These pouches can be acquired by killing Abyssal Monsters. To gain access, you'll need to complete the very easy mini quest 'Enter the Abyss'. There are a lot of drops and an NPC that teleports into Abyss is close to RS Gold Edgeville so you don't really need to be concerned about PK's. Make sure you don't take anything important. There are four types of pouches : Small rune Pouch that is unlocked at level 1 holds 3 essences Medium Pouch starting at level 25 Runecrafting carries six essences, Large Pouch can carry 9 at level 50 and Giant Pouch is able to carry 12 at level 75 Runecrafting.
To create runes on different altars, you'll need certain items associated with them. For instance, at the Air Altar you need Air Talisman or Air Tiara for crafting Air Runes as well as on Fire Altar it is Fire Talisman and Fire Tiara.
At the beginning of your RuneCrafting leveling journey you must complete Rune Mysteries quest which is necessary to begin crafting runes. Although this quest does not reward RC experience you can finish Enter The Abyss miniquest which will bring you from level 1 to 9.
It will also help you avoid the early stages of doing water, air and mind runes. If you've got 46 Magic 5 Construction, 46 Magic, And 5 Construction and The Grand Tree completed you can go on to complete The Eyes of Glouphrie quest that will get you all the way to Level 23, (if Enter the Abyss is completed too). If you don't want to go on quests, you're going to craft Air Runes.
If you haven't completed The Eyes of Glouphrie quest at level 9, you'll be making Earth Runes which are fastest at this level. Earth Altar can be found near the northwest of Varrock and is located just prior to OSRS Accounts the Lumberyard. To reach level 14 , you'll need to travel around 8 times. If you'd prefer to make more trips, you could use Varrock Teleports and Lumberyard Teleports, but it'll cost a bit more.