There are a number of different statuses that can appear in the status box on a contact.
Green – the user is available
Red diagonal lines – busy, meeting or on a call (will indicate “call”)
Orange - away
Empty – offline
Below, the user is shown as offline.
Below, there user is shown as away.
You can change your status manually by choosing from the drop-down list.
However, your status will update automatically so the most common usage would be to set yourself busy if you don’t want to be disturbed. For instance, your away status will be set after 5 minutes of inactivity.