The Sonar Beacon is a portable electrical device that can be activated to make the create a marker at its location on a Navigation Terminal, Sonar Monitor, or Handheld Sonar. It can be held by the users, in which case marker will be on top of the user, or it can be dropped to mark a specific location.
By putting it on either of your hands, the Beacon's GUI opens up. There's an on/off toggle check box and you can also type in how the marker should be labelled on sonar. After being turned on, the Beacon can be put anywhere in the inventory or dropped.