It's multiplayer stacking sats.
There is no coin, there is only plebs buying sats.
Stackchain is people buying Bitcoin, started with a 5 dollar buy back in July of last year. Each 'block' is 1 dollar more than previous 'block'.
So someone bought 6, then 7, then 8 etc.
It's been contained in a crazy long thread on the old blue bird app.
We are currently at block like 3047 over on twatter.
You don't have to buy a 'full block's you can buy any amount and post it.
Here is an example of the stackchain that was on twatter.