const FORCEUPDATE = String(process.env.UPDATESILENCE_FORCEUPDATE) == "true"

The dotenv module makes the process.env property available.
The name given after .env is the name of a "property" in the .env file that the dotenv module reads from.
If it doesn't exist, then process.env.WHATEVERNAME will return undefined, which then evaluates to String("undefined"), giving "undefined".

Since the comparison "undefined" == "true" is false, this makes FORCEUPDATE false.
However, if process.env.WHATEVERNAME has a value set, and if the value is true, then String("true") will return true, and the comparison will return true.