There are a few different ways to upload documents to SharePoint: Browser / Drag and Drop.
Browser:
If you are working with a document you have already saved locally, the browser is the easiest way to save your document. Simply locate the area in SharePoint you would like to save the file, select Upload, and upload the file from where it is stored locally on your machine.
Additionally, you can create files directly through the browser which will save to the library and you can edit files in the browser which will save to the library.
Drag and Drop:
If you are working with a document you have saved locally AND you prefer the drag and drop experience, you can upload files using File Explorer. This method is preferable for uploading large amounts of files at once.
- Launch File Explorer on your machine and locate the file(s) you have saved locally.
- Find the area of SharePoint in which you would like to save the file
- You can open SharePoint in the Browser or drag and drop within your file explorer if you prefer.
- Once in your desired location, select the file(s) you would like to upload by highlighting all files and drag/drop into SharePoint.
- Browser: When dropping into the browser, you should see a transparent square with the files as you drag and the area in SharePoint you wish to drop them will highlight. You will be shown a progress of the files as they upload in the top-level menu and a confirmation message they have uploaded once it is complete.
- When dropping within File Explorer, you can hover over your SharePoint section, allow the folders to expand, and then drop where you would like.