abount scatter demo SPACE_INVADERS_OWNER_PKEY

```
const signProvider = (buf, sign) => {
    return sign(buf, process.env.SPACE_INVADERS_OWNER_PKEY)
};

const username = this.scatter.identity.name;
this.eos.contract('invaders', {signProvider}).then(contract => {
    contract.scored(username, score, account.name, process.env.SPACE_INVADERS_OWNER_NAME, options)
});
```

Hi, What does SPACE_INVADERS_OWNER_PKEY mean here?