Online presence information in SharePoint allows users to see if other users are online or not. Imagine a document in a document library: Next to the authors name or modifiers name you can see if this person is online or not. If you have a question regarding this document you can easily send instant messages. This way you can get things faster done than looking up the user to get the phone number and to make a call.
The presence information is displayed by an ActiveX controls and therefor requires Internet Explorer and an Office version of 2003 or newer.
If you have an intranet you may want to use the online presence information together with Lync. If you have an internet facing website you want to disable the presence information.
How to enable or disable Online Presence Information
Select manage all web applications in Central Administration and select the web application where you want to disable online presence. Go to the person name actions and presence settings property to disable the online presence of a person.