They are used as a classroom resource to visually represent various areas of language. Cuisenaire rods are used in the Silent Way, a teaching methodology associated with humanism.
Example
Rods of one colour are used to represent verbs, another is added at the end to represent the past -ed form.
In the classroom
Many uses have been found for rods in the classroom. They can for example be used as realia, e.g. asking learners to compare sizes and to demonstrate prepositions, or as representations of concepts such as verb forms and word order.
Further links:
https://www.teachingenglish.org.uk/article/cuisenaire-rods-storytelling
https://www.teachingenglish.org.uk/article/cuisenaire-rods-language-classroom
https://www.teachingenglish.org.uk/article/rediscovering-silent-grammar