The pathway is a metaphor for this search. The speaker sees that the path is thick with weeds, symbolizing that truth is often difficult to discern because it is clouded with "weeds." Also, each weed is a "singular knife" meaning each weed is some obstacle to the truth - perhaps deceit, perhaps self-serving motivies, perhaps white-lies, perhaps lies by government, corrupt leaders, etc. (I'm extrapolating here).
The speaker also acknowledges that not too many are interested in seeking the truth, because there is evidence that the path has not been traveled much.
The poem ends on a pessimistic note because the speaker, too, decides that the path to truth is too treacherous with all those weeds, and he, too, decides to look for other roads.
See the links for further help.