StringTake

StringTake["string", n] gives a string containing the first n characters.

StringTake["string", -n] gives the last n characters.

StringTake["string", {m, n}] gives characters m through n.

Examples

First 5 characters:

StringTake["Hello World", 5]
(* "Hello" *)

Last 5 characters:

StringTake["Hello World", -5]
(* "World" *)

Substring:

StringTake["Hello World", {7, 11}]
(* "World" *)

Please visit the official Wolfram Language Reference for more details.