This is the 8872nd most frequent Japanese word.
横山
"横山" (Yokoyama) is a Japanese surname meaning "horizontal mountain."
In this sentence, '横山' is used as a person’s surname, addressing Mr. Yokoyama formally with the honorific 'さん'.
横山さんは教師です。
Mr. Yokoyama is a teacher.
Here, '横山' refers to a place name, likely describing a geographic feature such as a hill.
この横山の丘は景色が良い。
This Yokoyama hill has a good view.
In this context, '横山' again refers to a location, specifically the subject depicted in the painting.