。
The person painting the wall is my father.
Literal
Wall [object-を] painting person [topic-は] my father is.
壁を塗っている人 — the relative clause 壁を塗っている modifies 人: 'the person who is painting the wall.' は marks this person as the topic, and 私のお父さんだ identifies them. A classic example of Japanese relative clauses preceding the noun they modify.