This is a special type of Pass Through node which writes the values that pass through this node to the Script Editor history pane so that you can debug your graphs. You can choose to print all values that are passed into the In port or limit the output to a range of elements.
If you have more than one Log Values node in your graph, you can specify a special string in the Debug Print Tag parameter to uniquely identify which values are being logged. For example, if you set it to "Turbulize the Gravity" and pass the Turbulize value through the Log Values node to set the Gravity Force compound, all logged values from this connection will be bracketed with these messages:
INFO : 4000 - Begin: output for "Turbulize the Gravity" DebugPrintNode:
INFO : 4000 - End: output for "Turbulize the Gravity" DebugPrintNode.
The Out port must be connected as well in order to log and pass the values through.