‘Let’s Have a Yarn’ Means ‘Let’s Have a Chat’

In Australian English, Let’s have a yarn often means ‘Let’s have a chat’. Yarn originally refers to thread and can also mean a story; Macquarie Dictionary also records the verb sense of talking or chatting. The invitation usually feels relaxed and personal, perhaps suggesting an unhurried conversation over a cuppa, rather than something as formal as Let’s have a discussion.

For example: I haven’t seen you in ages. Let’s have a yarn after lunch. Do not combine the phrases into the unnatural have a talk yarn. Spin a yarn is different: it puts the focus on telling a story, sometimes an exaggerated one. Have a yarn may simply mean catching up and exchanging news.

https://www.macquariedictionary.com.au/heard-it-on-the-bush-telegraph/
https://dictionary.cambridge.org/dictionary/english/yarn