When Travis is sitting in the diner with the other drivers, they're telling him he should get a gun for his own protection. As we know, he definitely takes this advice… although he has more than his own protection in mind.
When his fellow cabbies are actually dishing out this advice, he seems kind of distracted, something the camera conveys. Particularly, Travis is entranced by the tab of Alka Seltzer fizzing in his glass.
(If you've never watched Alka Seltzer fizz in a glass of water, we totally recommend it. It's kind of like ye olde Pepsi-and-Mentos trick… except it can also help your tummy when it's upset.)
Why is Travis so mesmerized by the bubble action, though? Maybe this is because Travis is—like the Alka Seltzer—fizzing with anger and discontent on the inside? He's kind of at a simmer that will eventually come to a boil. The camera lingers on this image because it symbolizes Travis own inner state. While the cabbies are trying to talk with him and be normal and human and friendly, he's inwardly seething, and searching to find a target for his hidden rage.