。
Walking is good exercise for your health.
Literal
Walking [nominalizer-事] [topic-は] health [for-に] good exercise is.
事 is the kanji for こと — sometimes written in kanji when the nominalization feels a bit more concrete or abstract-conceptual. 健康によい is the standard 'good for health' pattern: に marks the beneficiary/standard (for whom/what it's good).