。
She likes going for walks by herself.
Literal
She [topic-は] alone [as-で] walk [for-に] going-out [nominalizer-の] [subject-が] is-fond.
~のが好きだ — the nominalizer の turns the verbal phrase 散歩に出る ('go out for a walk') into a thing that can take が; 好きだ is a な-adjective whose object is marked with が. 散歩 is the canonical noun for a leisurely walk, distinct from active 歩く ('to walk'). The に on 散歩 marks the purpose of going out, parallel to 買物に行く.