What are frills?
They are fancy, frilly things that you often find on curtains or dresses - like this:
But no frills?
This becomes an adjective to describe a product or service that is basic. The word became most synonymous with budget operators called no frills airlines like Ryan Air, Wizz Air and Easy Jet. Here many of the extras you normally get were removed to reduce the base cost of the flight to as little as possible.
Like this maybe: