Filling is the process you take to make something full. The bucket in the picture is being filled with water from a faucet.
When an animal or plant dies and is buried under the ground with the right conditions for thousands or even millions of years, it turns to stone. Later when we dig them back up, we call them a fossil. When something has been turned into a fossil it is fossilised. People have found fossils of all sized creatures, from tiny sea snails, to frogs, right up to enormous dinosaurs. Most of the creatures that people find as fossils are extinct - you can no longer see a living animal of that kind. The fossil in the picture is a trilobite.
A frill is a type of trim around the edge of something to make it look prettier. A frill is usually gathered on one edge and free on the other, forming a ruffle.
Gills are what creatures that live in water use to breathe. Fish gills are easy to see, between their head and their side fins. Other creatures have less obvious gills, that look like fine, tufty growths.
A grill is a metal grid that is placed over intense heat or open fire which is used for cooking food, typically meat.
A grille is a metal grate used as a decorative and protective covering, especially for a car or a fireplace.
Hills are small mountains, and have a much gentler slope than mountains do.
Ill is another word for sick, as in "I feel ill today". It may also be used before another word to make a negative phrase out of that word, for example "the child was very ill mannered" , or "she received nothing but ill will".
You kill something when you stop it from being alive. When a person kills another person on purpose, it is called murder. Killing might also mean stopping something from happening, for example "lack of money killed his plans to go to Europe".
A kiln is a special kind of oven designed for firing pottery or bricks. Some kilns are big enough to dry timber in, or for smelting ore into metal.