Instant messaging (IM) is a type of online chat which offers real-time text transmission over the internet.
Some of the benefits of using IM over other communication (email or phone) options:
You don’t have to open Skype for Business to have an IM conversation with people in your organization’s address book. You can start or accept an IM session with someone directly from Outlook on the web.
Microsoft provides extensive documentation on how to use this feature.