The Threshold Filter node, given an analog value on the Input connector, sends it to the Output connector if the value is included between the values specified by the two properties Threshold Min. and Threshold Max., otherwise it sends the value of the reached threshold.
It is possible to exclude one of the two thresholds by editing the Filter type property of the node.

 Behaviour of Threshold Filter with Filter Type set to //Minimum and maximum thresholds//

Behaviour of Threshold Filter with Filter Type set to Minimum and maximum thresholds