As Github official says “SSH keys are a way to identify trusted computers without involving passwords“, we have to bind ssh key generated at local machine to our github account. It is necessary to communicate with git repositories with ssh key.
If you are newbie to github, just walk following 3 steps to achieve it. If you forget ssh passphrase just like me, firstly delete existing ssh key binding from your githut account (Click avator at right top -> setting -> SSH and GPG keys -> delete), then generate and bind a new one.
Checking for existing SSH keys: https://help.github.com/articles/checking-for-existing-ssh-keys/
Generating a new SSH key and adding it to the ssh-agent: https://help.github.com/articles/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent/
Adding SSH Key to Github Account: https://help.github.com/articles/adding-a-new-ssh-key-to-your-github-account/
Github SSH Key (Mac) Reference: https://help.github.com/articles/generating-an-ssh-key/#platform-mac