Hello..! :)
By Variable Words, we know that they are those that undergo modifications in their form to express gender, number, degree, time, manner or person.
Example:
Girl (It is feminine singular)
Children (It is masculine plural)
Girls (It's feminine plural)
And these cause them to experience change or bending in their shape. They are the verb, the noun, the adjective, the determiner and the pronoun.
¿Doubts? On the comments. Greetings :D