If you mean when you make quotation marks with your fingers, you can use them when ever we would use ' ' when writing, to signify a 'controversial' word. They are usually used to show that the word is incorrect, an exaggeration, unsuitable, or maybe it's just a new or not well known word. They can also be used to show doubt or uncertainty. "He said he was 'happy'." Here, we would either doubt he was really happy, or we would be demonstrating that we're not sure what 'happy' means in this context.