You are searching for books in and you come across records with the link "online access." This means that the book you are looking at is available electronically.
There are multiple methods to access library content, depending on whether you want to download materials onto a device, or if you want to read off of the screen. Which method is best? It depends on what you would like to do.
Here is a breakdown of pro's (+) and con's (-) for both methods.
Unless you want to take the ebook with you where you don't have any internet, it's usually advisable that you simply read on the screen. If you create an account and add the book to your folder or bookshelf, you can return to it quickly, take notes on it, bookmark certain pages. All of that is possible without downloading the ebook.