This symbol indicates a connection.  Normally a connection appears as a yellow line, but if it's zero length it's replaced by this symbol:

Typically, this would appear with a surface mount pad on the same net as a powerplane to show that it's isolated from the plane.  To make the connection, double click to start a track, move away from the pad, then right click and 'End on Via'.  Vias added in this way may be shared with other similar pads.