There isn’t much information to go on here, but I found this: https://explore.postman.com/api/74/healthos is this the one you are referring to? I tried to import the collection and also saw this as well:
If it’s red it means the variable is unresolved so, generally, either they’d include an environment for you to import ( which doesn’t seem to be the case here ) or expect you to define a variable, in this case, host, with the appropriate domain or host for the API.
According to their documentation here: HealthOS they expect you to create an App before getting started but it looks like the link they included is broken, so perhaps they haven’t updated their docs recently.