You can identify matter as being a gas, a liquid, or a solid based on simple observation.
A gas has "neither constant volume nor constant shape." Gases expand to fill whatever space is available for them to enter, in whatever shape that space allows. Think of the hydrogen, carbon dioxide, and other gases in the air. Those gases fill the space in a room that is not occupied by furniture. When more furniture is moved into a room, there is less of the gas contained within that space.
A liquid has "constant volume but indefinite shape." Imagine putting eight ounces of water into a variety of containers. That amount of water might fill up a small drinking glass; it would fill roughly half of an empty soup can; it would barely cover the bottom of a large cooking pot. The volume (the amount of water) wouldn't have changed - it was eight ounces in every case - but the shape changed to fill the container in which it was put.
A solid has "both constant shape and constant volume." A solid object, such as a red clay brick, retains its shape and volume regardless of changes in temperature, surrounding environment, or container into which it is placed.