Many of the ebook collections to which the UMD Libraries subscribe have restrictions on how many users can access them at one time. Many also include digital rights management (DRM) software, which is embedded coding that limits the amount of copying and downloading allowed, and the amount of time during which a user can access the book. Once you are logged into the ebook you can often determine what these restrictions are by reading the introductory information which typically appears after the information about the book (title, auther, abstract, etc.)
Here ia an example from a Proquest Ebook Central title which explains that there is unlimited access, you can download up to 183 pages, you can check out the book for 21 days, etc.