What is the meaning of the poem called " History Lesson"?

This poem seeks, as the title indicates, to impart a history lesson to the reader. However, the structure of the poem makes it difficult for the reader, not knowing the context or the poet's racial identity, to imagine at first what the lesson could be. After all, the picture the poet draws of the photograph is one familiar to all family albums: a four year old child, standing on the beach in a bikini, smiling into the camera while her grandmother takes a photograph.
The poet, however, deftly switches the tone of the poem away from this idyll when she states that it is "two years after they opened this beach to us." Note the enjambment, the distribution of the lines, which create a break between the seemingly innocent picture and the revelation of what it means in the context of this black family's existence.
To the smiling child, there is no awareness of anything beyond the beach. The adult the child has become, however, cannot help but draw a stark comparison between this picture of her young self and a picture of her grandmother, years earlier, standing on a strip of sand marked "colored" because it was not permissible for black people to access the whole of the beach. This photograph, then, when compared to the older one, is a stark demonstration of how times have changed for the black community in America. It is also an indication that, for many children, as time advances they will never understand the difficulties their foremothers faced. In some ways, this is good for them, reflective of progress, but in others, it is important to know our history so that we never repeat it.

In her poem "History Lesson," Trethewey compares a photograph of her four-year-old self standing on a beach with a photograph of her grandmother as a young woman standing on the same beach. She deftly sketches the beach with a few vivid phrases: the "wet sand" under the child's feet, the sun flashing on the water of the "rippling Gulf," the small fish "glinting like switchblades" in the surf. On this "wide strip of Mississippi beach," her young self stands in a flowered bikini. It is an ordinary picture of a childhood seaside outing, until Trethewey adds the context:

It is 1970, two years after they opened
the rest of this beach to us[.]

This is not any beach: it is a beach that used to be segregated. Four-year-old Trethewey would not have known the importance of her position in the photograph; she was innocent of the "history lesson" that adult understanding now imparts. Only her grandmother, the person who took the photograph, knew what it meant for this small black child to stand on that beach. There is another photo, Trethewey reveals, showing her grandmother as a young woman standing on the same beach,


on a narrow plot
of sand marked colored[.]

Time has changed the circumstances of each photograph, from a society where racial segregation was the norm, to one where it was slowly being phased out. The adult Tretheway is 30 years past that snapshot of her four-year-old self, and one hopes that time has continued to move in the right direction since that picture was taken.