An interesting aspect of the "Filling Station" is that the speaker makes a quite ordinary sight/experience (passing by or patronizing a gas station) and, through the processes of induction and detailed observations, she derives a profound conclusion. In discussions of poetic explication, this is often called "finding the extraordinary in the ordinary."
The speaker describes the filthiness of the filling station in great detail. Although not an imagist poet, Bishop does create a poem that evokes a clear and detailed image. The details of this image are what make the poem effective. After criticizing the dirty aspects of the station, the speaker starts to make other observations that indicate the station is lived in, that it has a home atmosphere (in spite of the dirty functionality of a gas station). For example, she asks why there is a plant or a doily. The speaker finally concludes that the absent member of this station family, the mother, must have embroidered the doily. She continually refers to this mother as "somebody" because the mother never appears. All the speaker sees is the loving care this "somebody" has shown to make this overtly dirty place at least somewhat presentable.
Somebody embroidered the doily.
Somebody waters the plant,
or oils it maybe.
There is some subtle humor with the supposition that, because there is oil everywhere, this "somebody" might oil the plant. This takes the reader back to that stern criticism of the station's filthiness to present an even sharper contrast with the final conclusion that "Somebody loves us all." The speaker is very condescending towards the filling station family but entertains a communal epiphany at the end of the poem. If this "somebody," presumably the mother, loves her family, it can be seen in the details. One implication of this is a connection between watching and "watching over" with the motherly or guardian angel association.
Bishop's own mother died when she was very young. There is an interesting connection here between Bishop's own mother and the mother of the filling station in that they are both "absent" but one can see the detailed evidence of their love in the world.