Sequence-controlled polymers are long molecules made up of many smaller pieces called 'monomers'. These monomers are connected together in a certain order, like a string of beads. They are kind of like a long strand of spaghetti that has been arranged in a specific pattern. By carefully arranging the different monomers, scientists can make polymers with different properties that can be used for different purposes. For example, these polymers can be used to make materials stronger and more durable, like car parts and electronics.