From the start of the poem, Yeats is pretty up front about the fact that he makes a lot of meaningless small talk with the people he runs into on the streets of Dublin. He even repeats the exact same phrase, "Polite meaningless words" to talk about how boring and repetitive this process can be for him. But hey, the dude has to act polite, even if he's being totally phony.
Lines 5-6: Yeats says that he has passed people leaving their workplaces with a "nod of the head / Or polite meaningless words." Basically, everyone in Dublin would have known Yeats because he was a celebrity. And he's happy to say hi, but doesn't really care about these people or what they have to say to him. Nice.
Lines 7-8: Yeats doesn't always pass people with a nod of the head, though. Sometimes, he'll even go so far as to "linge[r] awhile" and say more "Polite meaningless words." So basically, whether Yeats stops to talk to you or not, his words are still going to be totally meaningless, like "Hey, how 'bout them Yankees?" Or whatever the Irish equivalent is…
Lines 9-12: And if he really, really wants to put in the effort, Yeats will even think of a funny "mocking tale" that will entertain people "Around the fire at a club." So yeah, he'll tell a funny story now and then. But it's still totally meaningless to him. That is until everything gets "changed utterly" by the Easter Uprising. Then Yeats realizes he might have to take these folks more seriously.