The WordPress Media Settings deal with the sizes of the images you upload to WordPress and how they are organized. Something you might not know is that when you upload an image to WordPress, WordPress automatically also creates images in the sizes as listed in the Media Settings. So when you upload one image, you actually end up with at least 4 versions of that image and sometimes more if a theme or plugin add custom image sizes.
WordPress will always keep a full-size version of your file however when the file is uploaded it will make a copy and resize the image according to the size listings as shown above. For thumbnails, it will create a 150px x 150px square image. WordPress will create a medium and large size version of the image as well.
WordPress resizes the images to help reduce load time and improve the visitor’s experience by displaying the image in the best size. This is not an exact size and the automatic resizing, especially the thumbnail, will remove parts of your image and in that case, you may need to do some editing and resizing before uploading the image to WordPress.
Some WordPress themes use different size images. If that is the case then you can update the size of the thumbnail, medium, and large images. Many themes will even add additional image sizes. So you may actually seem more options than appear on the screenshot above.
At the bottom of the Media Settings page is a single checkbox - Organize my uploads into month- and year-based folders. If this is checked, when you upload files to WordPress all files will be organized in folders, or directories, according to the month and year the file was uploaded. If unchecked, all files will simply be uploaded in the main uploads folder and not organized.
The main difference is how the images are accessed. For example, the URL of a file upload to the month and year folder would look like this: https://example.com/wp-content/uploads/2020/1/file.jpg. Otherwise, the URL of the file would be like this: https://example.com/wp-content/uploads/file.jpg. Notice the difference? This is a personal preference and does not affect how your WordPress works.