The opening stanza of "Sestina" sets the scene. It's a gloomy September day, and "rain falls on the house." Inside, an "old grandmother" and "a child" sit beside a "Little Marvel Stove" (an old-fashioned wood-burning stove) in the fading light of day.
Calling this light "failing" creates a slightly ominous tone, though nothing seems particularly out of the ordinary at first. The grandmother is reciting "jokes from the almanac," a book that contains weather forecasts, astronomical data, recipes, and so on, presumably having a nice time with her grandchild. It isn't until line 6 that the speaker hints at something darker bubbling beneath the surface: the grandmother is "laughing and talking to hide her tears." She knows something that the child does not, it seems, and is trying to protect the child from the pain of this knowledge.
Note how the speaker refers to these characters as "the old grandmother" and "the child" throughout the poem, rather than saying "an old grandmother" or "a child." The article "the" suggests that the speaker is talking about two very specific people at a very specific moment in time. Given that the poem was inspired by Elizabeth Bishop's own life, readers might envision the poet looking back on her younger self.
The poem's title also announces its form up front. A sestina is a complicated verse form consisting of 39 lines broken into seven stanzas: six sestets (six-line stanzas) followed by a final tercet (three-line stanza). The last words of each of the first six lines are repeated in each subsequent stanza (these repetitions follow a very specific pattern, which is discussed in more detail under the Form entry of this guide).
The poem is also written in a rough iambic tetrameter, meaning that lines contain four iambs (poetic feet that follow an unstressed-stressed syllable pattern: da-DUM). Here's the first line as an example:
Septem- | ber rain | falls on | the house.
This line is almost perfect iambic tetrameter, with the exception of the trochee in the third foot (a trochee is a foot that follows a stressed-unstressed beat pattern: DA-dum). The poem will contain many more variations throughout, its meter never totally settling into a perfectly predictable pattern.