。
Harry really is a fluorescent light.
Literal
Harry [topic-は] really fluorescent-light is [confirmatory-ね].
A slang idiom that's hard to guess from vocabulary alone. 蛍光灯 literally means 'fluorescent light' (or 'fluorescent lamp'), but used of a person it's a slang expression meaning 'slow on the uptake,' 'dim-witted,' 'slow to get a joke.' The image is of an old fluorescent tube that flickers and hesitates before finally turning on fully — compared to a person who takes a long time to catch on. Used teasingly or affectionately, not maliciously. This is the kind of vocabulary you won't find in a textbook but will hear from native speakers. The confirmatory ね invites agreement.