To use expiry date and batch codes on selected products you need to change the Use Expiry to either Yes or Optional. If set to yes you will need to enter these details when you bill the product or else it will not let you proceed. If you selected Optional that allows you to skip this if you want.

 

When a product is received from a supplier and it has Expiry set to yes it will ask you to enter the batch and expiry dates for a specific quantity. You could received 10 items but they might have two different batch codes.

When products have the batch and expiry entered this way it will be a drop down option when you bill the product so you don't have to type out each time.

 

There are setup options available in Options | Setup | Product. 

This will let you print the batch and expiry on bills and statements if you want.