The worse problem is when one wheel is pointing left and the other right. If the only problem was the car not rolling straight the driver could turn the steering wheel, and then it‘s straight. When the wheels are facing in opposite directions the driver can’t correct for this and actually may not be aware of it as it may not cause the car to turn, but with the tires (slightly) sliding to the side instead of rolling straight ahead, they will wear out much faster then they should.

A fluid describes something that can change shape to match the shape of its container, and can move.

A gas (like air) is actually a fluid, it can flow down a pipe just like liquids can.

The difference is gas expands to fill its container, while liquid only takes up a set amount of space. A glass half full of water will stay half full. If a glass was half full of air, the air would spread out to fill all of it with less-dense air.