5-Stars Silver on any completed Episode means you obtained the required amount of coins, but did not serve all customers.

5-Stars Gold on any completed Episode means you obtained the required amount of coins and served all customers.

TIP: if you are clearing and collecting your tips too rapidly, you might be getting overwhelmed by customers. Try to hold off until you're ready for a new customer to be seated. This also alows you to do a "sweep" that will give extra rewards.