Let's take a dictionary and add at the beginning a new page that associates each letter with the page that letter's first word is. Now, at the begin of each letter, we can create another page for the that letter and the next one in the word, and so on.
Now we can find out each word by following the pages we just added, letter by letter.
Each of these pages we are adding is called an "index". A Btree allows you to follow index after index until getting the item you are looking for. The number of indexes to follow grows with the number of items, but at a much slower rate.