In order to handle your trades securely, we need to authenticate you with Steam. We can't have everyone going around saying "I'm Robin Walker and I will trade you". Because not everyone has that name (we're estimating a hefty 53% of Steam users are not named Robin Walker). The below button will take you to a secure page on the Steam Community website where you can authenticate yourself by logging in. Steam will then redirect you back here telling us that you are who you are. We do not see your password in any part of this, as Steam will be handling all the authentication.
If you are unable to login, it may be a result of the steam community being down. Valve usually warns about planned downtime.