We often use dictionaries to help us with spellings, but a dictionary has another purpose as well: it gives the meanings (or definitions) of words.
A definition is quite hard to write. It needs to explain exactly what something is without being too complicated, but it needs to be specific enough so that it only describes one thing.
Can you see why this definition of a car is not a good one?
a motor vehicle with four wheels
The definition could also be describing a van or a small bus, so it is not specific enough.