Feifei and Georgie explain the modern slang phrase "say less," which expresses enthusiastic agreement without needing further explanation. They clarify that it is a casual, friendly alternative to "say no more" and is commonly used on social media. The hosts provide examples of using the phrase in everyday situations, such as accepting a break or agreeing to a plan. They also mention similar expressions like "I'm in" or "I'm sold" to show how learners can expand their vocabulary.