It most text editors if you select a piece of text and press any key (that’s not bound to some command) the selection would be replaced by the character bound to the key (or deleted if you press Backspace). In Emacs things look differently - by default typed text is just inserted at point, regardless of any selection.

Of course, there is a way to change this:

(delete-selection-mode +1)

Now when transient-mark-mode is also enabled (it is enabled by default since Emacs 23) any typed text replaces the selection if the selection is active.

Personally I find this behavior more useful than the default one. That’s why it should be no surprise that delete-selection-mode is enabled by default in Prelude.