Fetching details from the dictionary...

world; society; public
life; lifetime
age; era; period; epoch; generation
reign; rule
the times
world (of existence)
「代」 has other readings with different meanings.
移り変わる世の中で大切なものを守りたい。
I want to protect what's important in this changing world.
平和な代が長く続いてほしい。
I hope this era of peace lasts a long time.
世の中には不思議なことが多い。
There are many mysterious things in the world.