If something is important, things can still happen without it.
If something is essential, it is necessary. Nothing can happen without it.
Bob is an important member of this team= Bob is valuable, but the team can still accomplish its goals without him.
Bob is an essential member of this team= If bob is not here, the team cannot do its work.
Lemons are an important ingredient in this cake recipe= if you don't add lemons, the cake will taste bad
flour is an essential ingredient in this cake recipe= if you don't add flour, the thing you make won't be a cake at all.