watsapp double tick

It know what double tick means. Just to ask, when exact the double tick be active?

1) Does it active when i access into watsapp or active when i enter the person conversation?

2) Does it show double tick if the message pop out as notification and i press close?

3) How to stay invisible in watsapp, is it possible?

Thanks!!!>>

It know what double tick means. Just to ask, when exact the double tick be active?

1) Does it active when i access into watsapp or active when i enter the person conversation?

2) Does it show double tick if the message pop out as notification and i press close?

3) How to stay invisible in watsapp, is it possible?

Thanks!!!>>