Coherence refers to how well ideas flow and connect smoothly from one sentence or paragraph to the next in a logical manner. Coherence in writing can be achieved through techniques like using repetition of key words, transitional expressions, pronouns, synonyms, and parallel structures to link ideas and create a smooth flow of information for the reader. Maintaining coherence is important at both the paragraph and whole text level to ensure the reader can easily understand and follow the overall argument or discussion.