"Be sure to take it slow," is used to warn someone. You could altenatively say, "Be careful." Some examples of usage: Person A: "I am a beginner at ice skating." Person B: "Be sure to take it slow. You could fall and hurt yourself." Person A: "I met this new girl yesterday, and I rellay like her." Person B: "Be sure to take it slow. You were just in a relationship."

It means "do not rush (something)"