<<<Verb>>> s are <<functions>>, which apply to arguments (nothing strange here).

<<<Adverb>>>s are like <<decorators>>, which modify functions.

In the line +/ 1 2 3 4, the verb is + (sum), the adverb is / (insert between arguments) and the arguments is the list 1 2 3 4. This evaluates to the same line as 1+2+3+4.

Are defined using verb‍s as base and produce other verb‍s, like the Atop conjunction (@).