To 'sign in' on your email or a site like miniclip.com and other websites you need an account for.
For an email, to 'sign in', you put in your email address, and your password, and then you click the sign in, or Log in, button. Another way to say sign in, is Log in. The opposite of Sign in/Log in, is Sign Out/Log Out.
"What are you doing?"
"Signing in to my email"
"I am typing in my email and password and then going to press a button"
"Cool, can I 'sign in' after you?!"