When a living thing dies, it usually decays (disappears). Many times though, dead organic life will be captured in a substance that preserves it. When this happens in a substance that can hold it for hundred and even thousands of years, we refer to it as a fossil. The word "fossil" even means "dug up". Studying fossils help us understand past life. Some fossils can also be just evidence of past life, like a preserved footprint or an egg. When we put a bunch of fossils together we can get a pretty good idea of what life was like in early times.
The worksheets spend a good deal of time on the fossil record over time periods and how fossils are used as a tool to understand evolution.