Several of the functions for minibuffer input have an argument called initial. This is a mostly-deprecated feature for specifying that the minibuffer should start out with certain text, instead of empty as usual.
If initial is a string, the minibuffer starts out containing the text of the string, with point at the end, when the user starts to edit the text. If the user simply types RET to exit the minibuffer, it will use the initial input string to determine the value to return.
We discourage use of a non-nil value for initial, because initial input is an intrusive interface. History lists and default values provide a much more convenient method to offer useful default inputs to the user.
There is just one situation where you should specify a string for an initial argument. This is when you specify a cons cell for the history argument. See Minibuffer History.
initial can also be a cons cell of the form (string . position). This means to insert string in the minibuffer but put point at position within the string's text.
As a historical accident, position was implemented inconsistently in different functions. In completing-read, position's value is interpreted as origin-zero; that is, a value of 0 means the beginning of the string, 1 means after the first character, etc. In read-minibuffer, and the other non-completion minibuffer input functions that support this argument, 1 means the beginning of the string, 2 means after the first character, etc.
Use of a cons cell as the value for initial arguments is deprecated.