The borrowed feature and collection
The borrowed feature is pretty straight-forward but as the saying goes: there are many different ways to skin a cat. (Even though Bruji loves cats and would never do such a thing...)
Marking an entry as borrowed
or
or
Filling out the 'Borrowed by' sheet
Returning an entry
or
or
Using the Borrowed History feature
Once an entry has been borrowed it automatically receives a borrowed history which shows you who borrowed the entry on what date and when it was returned.
The 'Entries borrowed by...' feature
This feature allows you to see at a glance the history of a person's borrowing habits.
The status banner over the cover image
When you add an entry to the borrowed collection via any of the ways described above, not only will you get a mark in the table view to let you know of its status, but you will also see a red banner over the cover image of the entry that reads "Due back in x days".

If you would prefer not to have that banner over the cover image, go into your Preferences/Style and check the box next to "Don't show status banners over cover images".