The IELTS public band descriptors for speaking state that in order to achieve band 7 or above for Lexical Resource (LR - vocabulary), then you should use some less common vocabulary.
It's so hot today. I'm parched!
In this sentence, parched is an example of less common, or low-frequency vocabulary. If you use a word like parched, the IELTS examiner will begin to consider you as potentially band 7 or above.
Band 7 candidates, even band 8 candidates, might not know parched. Perhaps it is new to you today?
An example of a typically band 7 low-frequency word might be plummet:
In recent months, sales have plummeted.
Sometimes one form of a word is frequent, while another form is less frequent.
Notice that in each of these examples, the low-frequency word form is the noun form! In a lot of academic writing, the noun form is more useful than other word forms, because it can be strategically located at the beginning of a sentence in what is known as theme position.
If you are scholar preparing for study abroad, then you have probably acquired a large vocabulary of low-frequency items related to your academic specialisation. If the examiner asks questions related to your field, use this vocabulary!
It would be fair to describe the following as band 9 items:
An awareness of the frequency of vocabulary items comes through reading. When you read a novel, and you see a word often, then that word is a high-frequency word.