。
She writes beautiful characters.
Literal
She [topic-は] wonderful characters [object-を] writes.
字 (じ) refers to written characters or handwriting — the sentence compliments her penmanship specifically. Rather than using an adverb like 上手に (skillfully), the quality is expressed by modifying 字 directly with 素晴らしい: she writes 'wonderful characters,' meaning the physical form of her writing is beautiful. This kind of compliment is common in a culture where handwriting quality — especially brush or pen — has traditionally been considered a reflection of character.