Instead of uploading 1 file at a time to SharePoint, you can upload multiple files either using the Upload Multiple Documents button under the Upload Document button on the toolbar of your SharePoint document library, or using Explorer View. However, when you upload multiple documents, they may remain checked out to you. To check in multiple documents at once, you can use the Content and Structure view (which also is capable of doing many other powerful things that I'll elaborate on in later blog posts).
To check in multiple files:
1. Under Site Actions, click Site Settings.
2. Under the Site Administration section, click Content and Structure.
3. In the left pane, click the document library where you want to check files in.
4. In the right pane, click View: Default View to show the dropdown menu, then click Checked Out to Me. You should then see all the documents checked out to you.
5. Select the files you want to check in.
6. On the Actions menu click Check In. It will process for a moment depending on how many there are to check in, then you can return to your document library and should see they are all checked in.
Feel free to explore the other capabilities in the Content and Structure view. I'll be following up this post with another post on some of the other uses and features of the Content and Structure view.