what you see in the painting is the subject matter of that painting. it is the totality of the picture/ the picture in totality or the focus of the picture.
eg. if a painting is showing a dingy room full of people in tatters, eating their meal, you could say the subject matter was Poverty.
however, a lot depends on the interpretation of the viewer. a person may feel that the focus is on eating hence the subject matter is mealtime....
many artists paint a series of paintings based on some specific/broad subject. that gives another indication about the true intent of the artist & subject matter of painting.

then the genre also helps decide the subject matter. still life, portraits, fantasy, surrealism, land & seascapes, etc. ...
the technique can also contribute - charcoal sketches, oils, waters, .....
hope this helps
all the very best