The routers Reset button is usually located at the back of the router and is clearly marked. It is a small button usually sunken into the modem to prevent pressing it by mistake.

What it will do:

It resets your modem to the Factory Defaults. Resets your password, adsl account settings and IP address of the modem.

When to do it:
If you cant remember your ADSL routers password you can safely reset your modem to the default values.

WARNING: Remember to make note of the default routers IP address and login details (username & password) before reseting.
Remember if the default IP address is different from the current one you will have to change your computers IP address to the same range as the routers.

Also have your ADSL account details on hand.