So : "être prudent" is "TO BE careful" using the infinitive form of the verb. "sois prudent" means "(you better) be careful" in an imperative mood, like you'd give an order.

The verb in the phrase "sois prudent" is in the imperative form, therefore you'll use it when giving orders. "Sois" is subjunctive too: "J'espère que tu sois prudent" = "I hope you'll be prudent". "Être" is infinitive, "être prudent" means "to be careful": "J'ai besoin d'être prudent" = "I need to be careful". I hope we can help each other, sometimes I need help with English.