。
She has lovely handwriting.
Literal
She [topic-は] writing [subject-が] skillful [is-だ].
字 (じ) means 'character, letter, handwriting,' depending on context — here it's specifically her handwritten characters. Japan has a deeply ingrained appreciation for good handwriting (字がきれい / 字が上手) — children practice 書道 (calligraphy) from elementary school, and a beautiful hand is a valued personal accomplishment. 上手 is a な-adjective; the [topic]-は + [aspect]-が + [adjective] structure ('as for X, Y is good') is the standard frame for ascribing skills or qualities.