。
She gave me a long letter.
Literal
She [topic-は] me [to-に] long letter [object-を] gave.
くれる is the inward-directed giving verb, used when something flows toward the speaker or the speaker's in-group. The same event would take あげる ('give outward') if the speaker were doing the giving, or もらう ('receive') if the speaker reframed it as something they received. This three-way contrast — あげる/くれる/もらう — is the heart of the giving/receiving (授受) system, and getting the perspective right is one of the trickier intermediate hurdles. The detail 長い手紙 quietly signals that the letter took effort to write, which in pre-email Japan carried real social weight: handwritten letters with particular care over brushwork and seasonal greetings were a recognized form of devotion.