In this speech, Truman is essentially arguing that the US needs to be the policeman of the world along, perhaps, with being its banker.  It has to be prepared to offer assistance to any country that is in danger of having a totalitarian regime imposed upon it.

Truman argues that it is in the interests of the United States to help Turkey, Greece, and any other nation that is in such a situation.  This is so, he says, because the existence of totalitarian regimes poses a threat to world peace.  As Truman says in the speech, the US must be

willing to help free peoples to maintain their free institutions and their national integrity against aggressive movements that seek to impose upon them totalitarian regimes.

This is the role Truman envisions for the US.