From this portion of Banquo's question, put to Macbeth when the pair of them see the Weird Sisters for the first time, the audience might begin to gather how skeptical and intelligent Banquo is. His first instinct, upon sighting the strange women on the heath, is to ask questions, to try to figure out what they are and from where they might have come. Banquo actually asks a number of questions of the sisters and begins to make observations of them. This response indicates that he will not simply accept anything they say without question.
The first thing Macbeth says when he sees the Weird Sisters is "Speak if you can." He wants to know what they are, but his response is a great deal less detailed than Banquo's is. Banquo's initial questions and desire to understand who and what these women are presages his later skepticism of their statements. Macbeth, on the other hand, is much more likely to simply accept.
Banquo is referring to the Weird Sisters as he claps eyes on them for the first time. Straight away, he can see that there's something unearthly about their appearance. In fact, they don't even look as if they belong on the planet. Yet there they are, as big as life, standing upon the wind-swept heath as they get ready to deliver their fateful prophecy.
That Banquo should be the one to comment on the witches' appearance is significant. He understands immediately, even if Macbeth doesn't, that there's something dark and malevolent about these strange women. Banquo could be accused of being shallow by judging the Weird Sisters on their appearance, but it turns out on this occasion that you really can judge a book by its cover.
As the play progresses, it becomes clear that Banquo's initial assessment of the witches was correct and that they represent trouble with a capital T. Banquo is able to discern the witches' evil as he subscribes—like most of the play's audience, no doubt—to prevailing norms of good and evil. Macbeth, on the other hand, as subsequent events will show, is much more ambiguous in his relationship with society's notions of what's right and wrong.
There is a broad interpretation possible for so short a quote.
Banquo is describing the witches as he and Macbeth happen upon them. The full quote:
"What are these, so wither'd and so wild in their attire, that look not like the inhabitants o' the earth, and yet are on't?"
implies that Banquo is so struck by their appearance that he suspects they are not human. By "withered" and "wild" we might presume that Banquo assumes humans, or perhaps women in particular, to be better looking and better kempt; we might even go so far as to assume Banquo believes there is a limit to human ugliness and behavior, and that the witches exceed it.
The audience's impression of Banquo from this might depend largely upon the identity of that audience; an audience composed of feminist academics would interpret this very differently from a group of 16th-century English farm workers. We might think that Banquo is a chauvanist who has committed a capital sin by placing his own conceptions of beauty in conjunction with what he considers to be human, perhaps leading him to interpret the "Other" as nonhuman and therefore not worthy of human rights. On the other hand, Shakespeare needed it to be clear that the witches were not "earthly" beings, and therefore not part of what we would consider the natural or reasonable world; it is common in literature for appearances to be linked to motivations or thematic points in a story, and the witches' unusual appearance, i.e. being "inhumanly ugly" is meant to strike the audience with a sense of unease and mistrust. Indeed Banquo goes on to be a voice of reason to Macbeth, and it is he who sees through the seductive veil of the witches' prophecies to the dark forces they represent; we might argue that he knew this from the start, as evidenced by the quote in question.