The "onset" is the initial phonological unit of any word (e.g. c in cat) and the term "rime" refers to the string of letters that follow, usually a vowel and final consonants (e.g. at in cat). Not all words have onsets. Similar to teaching beginning readers about rhyme, teaching children about onset and rime helps them recognize common chunks within words. This can help students decode new words when reading and spell words when writing.
Why teach about onset and rimes?
They help children learn about word families, which can lay the foundation for future spelling strategies
Teaching children to attend to onset and rime will have a positive effect on their literacy skills
Learning these components of phonological awareness is strongly predictive of reading and spelling acquisition