What is mean RGX discount level? And how do I determine the RGX discount level as it is on the list?

How can I calculate discount with RGX?

RGX tokens represent the discount rights attached to the later RGE TGE and should not be confused with the RGE utility token, which will fuel the Rouge Network. For example, the RGX8 tokens will give a 8x times bonus distribution of RGE tokens during the main TGE. Meaning that one could use 1000 RGX8 to buy for 1 ether the equivalent of 8 ethers of RGE tokens during the TGE (at determined ETH price). I hope it will help you to understand more about the RGX lol.

What is actually the main purpose of the platfrom?

The Rouge Project aims to implement a protocol — named Rouge Network — as a global, trustless, permissionless and decentralized coupon platform between three types of actors: brands, the general public and content creators or publishers. (Source website)

By doing a lot of marketing, the project will slightly become famous. I think, sponsoring crypto events is one of best way to advertise

I just want to inform you that the airdrop is live here https://tokendrops.com/airdrop-detail/rouge/32  

What is the purpose of airdrop? does it mean a lot for a project?

I joined the airdrop too, the tasks are pretty simple though

To be an investor must definitely have a large enough funds to buy tokens in large quantities. And for more information about pre sale you can see it on the website.