If you've ever cooked using a recipe, you know that the best recipes tell you what you need to do before you start cooking. So when you're baking cookies, the first line of the recipe should read: "Preheat oven to 375 degrees" instead of telling you after you've finished making the dough that you need to cook it in an oven at that temperature. Similarly, the ideas in a piece of writing should follow one another in a logical sequence so that the reader knows how to make sense of what comes next. Sometimes they may even be able to predict what comes next. Or, they may be led to wonder about something just in time to have it explained to them. This is structural organization.
Organization also happens on the sentence level in the form of transitions. Transitions help the reader see the structural organization by using words and phrases to make connections between ideas. Other tips for organization: Have one main idea in each paragraph, and make sure that everything in that paragraph (that is, the development) has to do with that main idea. When you start on a new idea, let the reader know by starting a new paragraph. Make sure that each of these main ideas has to do with the thesis or topic of the essay as a whole.
Organization can be the hardest part of writing because in our minds, ideas are interconnected in a kind of three-dimensional Web, but in writing, they have to come one after the other. Outlining can help. You can also try writing down each main idea on an index card and shuffling them around into different patterns until you find one that works. Or you can use a word-processor to do the same thing with chunks of text.