The choice of license terms involves decisions about what, if any restrictions the author wants to put on what people do with the software.

If you want to make no restrictions at all, you should put your software in the public domain. An appropriate way to do this would be to include something like the following text at the head of each file:

Placed in public domain by J. Random Hacker, 2003.  Share and enjoy!

If you do this, you are surrendering your copyright. Anyone can do anything they like with any part of the text. It doesn't get any freer than this.

But very little open-source software is actually placed in the public domain. Some open-source developers want to use their ownership of the code to ensure that it stays open (these tend to adopt the GPL). Others simply want to control their legal exposure; one of the things all open-source licenses have in common is a disclaimer of warranty.