http://www.textcnaelb.com chmod -- short for "change mode" -- changes a file's permissions, or "mode". You can make a file readable, writeable and executable for the file's owner, group and world ("other"). The format is chmod (permissions) (file):
samizdata# chmod 777 /home/dave/file.txt
Permissions can be defined numerically or symbolically, but there's more info on that at the permissions page.