The Terminate action allows you to terminate a contact's participation in a Journey. See Fig 1 below. 

Fig 1

All Journeys will have a default Terminate action node as the final node (you cannot add another node after this default Terminate action). However, there are scenarios where you may want to add another Terminate node earlier in the Journey's structure.  This would usually occur as an action node in a Conditions action, such as the Yes/No Condition or the If/Else Condition. See Fig 2 below.

Fig 2