StellarStates of Matter
From Units 1 to 7 in AP Physics 1, you have only been dealing with 1 state of matter, solids. At the molecular level, solids have both a fixed shape and a fixed volume, meaning that the molecules are locked in place relative to one another.
However, Unit 8 deals with fluids, which can be broken down into 2 states of matter: liquids and gasses. The one important property of fluids is that they have fixed shape, meaning that they can adapt to the shape of their container. The molecules are not locked in place, and are instead free to move. Gasses are differentiated from fluids as they have fixed volume, unlike liquids, which have a fixed volume. For AP Physics 1, Unit 8 will mostly deal with liquids, with gasses being covered more in AP Physics 2.