installing forum

Hi,
I've followed the forum installation instructions & got to:

To finalize the installation all you need to do is to copy and paste the text in the text box below into a file called config.php and then upload this file to the root directory of your PunBB installation. Make sure there are no linebreaks or spaces before <?php. You can later edit config.php if you reconfigure your setup (e.g. change the database password or ).

The cache directory is currently not writable! In order for PunBB to function properly, the directory named cache must be writable by PHP. Use chmod to set the appropriate directory permissions. If in doubt, chmod to 0777.

The avatar directory is currently not writable! If you want users to be able to upload their own avatar images you must see to it that the directory named img/avatars is writable by PHP. You can later choose to save avatar images in a different directory (see Admin/Options). Use chmod to set the appropriate directory permissions. If in doubt, chmod to 0777.

this is the 1st time i've tried to do this.

How do i set the cache & avatar directory to php writable?  what is chmod ?

Thanks Nick.