The title helps readers understand what's happening here: the speaker is in a crowded subway station, where people appear like "apparitions"—or ghostly, fleeting images—as they pass by.
On the one hand, this line seems pretty simple: with so many people walking by, one face after another pops up in the speaker's field of vision. Yet the word "apparition" gives these faces a supernatural, ethereal quality, as if the speaker isn't quite sure if these people even exist. Thanks to the poem's title, readers can assume that the speaker's crowded surroundings are indeed real, even though something else will take those surroundings' place in the following line. "Apparition" also suggests something momentary—that the faces pop up in the speaker's field of vision and just as quickly disappear.
The word "faces" stands also stands out. Pound could have used "people," or otherwise drawn attention to entire bodies rather than just faces. The word "faces," however, accentuates the degree of monotony and blurriness in the image of the crowd.
Think about it this way: people's faces are usually their most distinguishing characteristic, or the primary way that people tell one human being from another. If, in this metro station, everyone's face shares the quality of an "apparition," then the speaker has detached themselves from the scene by describing other people as such. The speaker seems to view the crowd as one ghostly body, not caring to pay attention to its many details. This distancing also paves the way for the seemingly abrupt transition in the next line, as though the speaker has already begun thinking about something else.
Read in a different way, the use of "apparition" suggests something deeply spiritual and moving about this crowd. The speaker is having a sort of vision or spiritual experience while looking out on all these people who pass before the speaker's field of view for just a moment. Perhaps this reflects the ultimate unknowability of other people, or the speaker's sensation of being overwhelmed by the sheer number of different lives rushing past in an instant.