I'm sure that you know what 'strong' means, and know that it can refer to people and things in many different situations. 'Sturdy' means strong in the sense of solid and durable, and unlikely to break. For example, if you are going climbing in the mountains you will need sturdy boots. If you are going to send something heavy in the post, you will need a sturdy box to put it in. 'Sturdy' always means 'strong'. 'Strong' doesn't always mean 'sturdy'.