Meaning and local context
In Liverpool, “boss” means Excellent, impressive, or very good. A classic positive Scouse adjective.
Example in context
A local example is “That match was boss.” In plain English, that means “That match was excellent.”
That match was boss.
That match was excellent.
How to interpret it
The surrounding sentence is important when interpreting “boss”. Use the regional sense above when the situation matches the example instead of relying on the word or phrase in isolation.
Tone and caution
Outside Liverpool, listeners may first understand a manager. Because this is informal regional language, tone and situation should guide whether you use “boss” yourself or simply recognise it when someone else uses it.