i would say it is safe enough during the day. you can explore the places you want to, with no danger. but at night especially after 9pm it would be a lot better if you do not wander around all alone.

It is not generally unsafe but there are strange people sometimes, in the center as well as the “nightlife district” Jungbusch. Be careful at night, maybe take a taxi to get home if you are alone. Probably like any other bigger German city.

Since the main street in the city and the main station is pretty open spaced I've always felt comfortable, even at night. There are weird people once in a while but they have always been harmless. Other areas like the Neckarwiese and Neckarstadt-West are known for being a bit more dangerous especially because of the drug use there.