Lightening Source, part of Ingram, is an on-demand printing business. They work with publishers to get your book in print. Most bookstores order from LSI. Publishers can order boxes of their books.
It is not the only on-demand printing business, of course. It does, however, have a lot of connections with those people and businesses where you want your book to be known.
Getting your book onto Lightning Source is not terribly complicated. Getting your book ready to upload can be complicated, however.
You'll need to create three documents: front matter, book, and back matter. There are requirements concerning margins, mirror pages, and page count. And you must format your book to the page size you've chosen for your book. Unless your book is going to be 8.5X11 inches, don't submit that size of a page, even though it is probably the usual size document you work in. If your book is, for example, 5.5" X 8.5", then the pages you submit must be set at that size.
Once the three sections are created, you need to combine them into one document then create a pdf. All fonts used in the combined file MUST embed in the pdf. By "must" I mean if a font does not embed, you must figure out why and repeat the process until the fonts embed.
If this is your first book, once you've uploaded both the book and the cover, you must order a proof to review.
When you get your proof, read it carefully. Look for any mistakes or goofs. If you find mistakes, correct them then go through the process of creating a pdf of the new version.
Then re-upload. But make sure you've caught and corrected everything. Otherwise, you'll have to go back and do it all again. Each time you re-upload your pdf files (whether it's the book files or the book cover file), you pay a fee for doing it and for receiving another proof.
That will change with the second book, though. But the first one is a learning experience - and learning costs.
Hopefully, there were no mistakes from the beginning and you only had to upload the files once.
But remember, Lightning Source works with publishers. Your publisher may do all the formatting for you. Or they may expect you to submit it to them ready to go to Lightning Source.