IYSS is an acronym used as a response when someone is not entirely convinced of what the other person is saying, but they do not want to argue. It is used to express acceptance or resignation, even if the person does not agree with what is being said.
IYSS is a relatively niche abbreviation, and it is not as widely used as some other texting abbreviations. It is more commonly used in casual contexts, such as texting with friends or family.