A written instruction is usually a direction or order. 'Take out your books', 'Underline the key words', 'Take the dog for a walk', 'Shut the door!' - you will be very familiar with instructions. Sometimes instructions need to be much longer than just one sentence, however. If you are telling someone how to do something a bit complicated, you might write it down in a set of instructions.
See how well you know the difference between instructions and other types of writing by trying this English quiz.