What is the meaning of Sign_on?

To join something, after signing.

To commit oneself, as to a project, a goal, an organization, a cause.

To begin broadcasting a radio or television signal, usually at the beginning of a broadcasting day and after being off the air for several hours.

To log on; to start using a computer, radio, or other multicast device; to start talking.

To apply to receive unemployment benefits.

Nonstandard spelling of sign-on.

Source: wiktionary.org