To quote the GST act, “mixed supply” means two or more individual supplies of goods or services, or any combination thereof, made in conjunction with each other by a taxable person for a single price where such supply does not constitute a composite supply.

You can use the above link to understand what principal and composite supplies are.

For clarification

• A supply of a package consisting of canned foods, sweets, chocolates, cakes, dry fruits, aerated drinks and fruit juices when supplied for a single price is a mixed supply.
• Each of these items can be supplied separately and is not dependent on any other.
• It shall not be a mixed supply if these items are supplied separately;

Rate of GST on Mixed supply:

The rate of GST on the entire bill shall be the highest tax rate applicable in any one item of the bill. For example, when chewing gums (28%) and chocolate toffees (18%) are sold together at a single price, the higher rate, being 28% shall be applied to the entire amount of the bill.