Conditional clauses are sentences that express a condition and its possible outcome. They often use the word "if" to introduce the condition. For example, in the sentence "If it rains, we will stay indoors," the outcome of staying indoors depends on the condition of rain. These clauses can be categorized into different types, such as real, unreal, and mixed conditionals. Real conditionals refer to situations that are possible or likely, while unreal conditionals describe hypothetical scenarios. Understanding conditional clauses helps in expressing possibilities and consequences clearly in communication.