Well, I don’t know whether there is a specific meaning of “sandbox” in the context of FS2020 - where did you pick up that term? Here in the SDK forum? - but I can give you a general definition (in the context of software / development):
A “sandbox” is a “space” within which a given process - or an entire application - is confined. So don’t just think of a “separate computer not connected to other production systems”: it is really an “address space” which the “process” may access - but nothing beyond!
Do you trust all those sources? Some yes, some no (the screenshot is from a “script blocker” called uMatrix, in case you wondered: a Firefox extension)… but your browser trusts none of those sources really!
I hope that helps