Describes a person's physical health or constitution as being strong and resilient. Often used in formal or written contexts.
My grandfather is 90, but he is very robust.
To build a strong body, exercise and nutrition are important.
強健 means robustly healthy, the opposite of 弱体 in the sense of a person's weak physical constitution.
Compound of 強 (strong) and 健 (healthy). The exact historical derivation is uncertain, but the combination naturally conveys robust health.