Let's break down this resource in the most simple way I am able to do. RAM is storage, think of it as a bookshelf or a harddrive on your computer. So anything you want to store on the blockchain, you will need to buy storage space (bookshelfs), that are big enough for everything you want to do.
Each time you get new tokens, or create new NFTs, upload a smart contract or list NFTs on a marketplace, you need to put that into your bookshelf. It will be there until the listed NFT is bought, or you burn those NFTs.
If you intend to upload a smart contract on an account, you will have to buy a rather large bookshelf, or rather an entire book store, so you have enough space for everything you intend to do.
I hope that this very simple explanation make sense and that you understand the purpose of RAM. Worth mentioning is that each time you buy and sell ram, you have a 0.05% fee with that transaction.
Where is RAM actually stored?
When you buy RAM on the WAX Blockchain, what you actually buy is storage in the RAM of the servers running the WAX Blockchain. These are operated by the guilds you can see in this list on bloks: https://wax.bloks.io/. There is a real world limitation on how much RAM can actually be accumulated on machines for users on WAX to buy, so long-term this is as scarce at the tempo of technology develops.
At the time of writing, the MAX capacity of WAX is locked at 128gb, this is however likely going to be changed with RAM inflation very soon. RAM inflation means that each block on WAX we add more RAM available on the blockchain.
What is the main usage of RAM
Currently majority of RAM is occupied for WAX accounts. Each account created is using RAM, as they are stored on the blockchain. Another big RAM consumer is obviously NFTs, each NFT that is created takes a few hundred bytes of RAM. Which quickly adds up when you have over 80 million created NFTs.
Big DAPPs also need a big amount of RAM for entries into their tables in their smart contracts. This is the biggest cost associated with running a successful dapp on WAX.
RAM is a resource, but more like a token
The main difference between the RAM resource and CPU/NET, is that CPU/NET is just you staking the WAXP tokens and receiving those resources. But for RAM, it is actually you buying a different token for your WAXP tokens. This has a cost associated with it of 0.05%, as mentioned before. But it also means that if you sell your RAM later, you might get more WAXP than you paid, or less. All depends on the market price of RAM and the current supply.
Next, let's break down CPU and NET!