Cohesion refers to how well linked together ideas expressed in clauses, sentences and paragraphs are. It is an important assessment area in the IELTS writing exam. Cohesion is often achieved through the use of cohesive devices. The following sentences are not well linked together:
My name is Simon. I am twenty years old. I am already married.
Adding basic cohesive devices changes this:
My name is Simon and I'm twenty years old. Although I'm only twenty, I'm already married.