。
She kept him waiting for a long time.
Literal
She [topic-は] him [object-を] long [genitive-の] interval made-wait.
待たせた is the causative-past of 待つ ('wait') — 'made (him) wait.' The を marks the person made to do the action; this is the standard transitive-style causative pattern (when the original verb is intransitive, you typically use を for the causee). 長い間 ('for a long time') is an adverbial duration phrase — note that Japanese expresses durations like this without an explicit 'for' preposition, just placing the phrase in front of the verb.