These lines come from Section 52 of this poem which is the final part. The lines you have identified must be read in the context of the rest of this poem for us to be able to understand them. This section of the poem begins when the speaker sees a hawk in the air that seems to "accuse" Whitman for talking too much:
The spotted hawk swoops by and accuses me, he complains of my gab and my loitering.
In response, the speaker utters the two lines you have quoted. Whitman clearly feels that he is connected to the hawk and to nature, as his voice cannot be translated. The second image he gives us relates his voice to a "barbaric yawp" or an animal sound that has no link to human speech. The fact that the speaker says this about his voice shows how he belives that he is intimately part of nature and its essence and power. These lines therefore focus on the strength that the speaker feels he has within him and the indomitable spirit that expresses itself through his voice.
I too am not a bit tamed, I too am untranslatable,
I sound my barbaric yawp over the roofs of the world. (1332-3)
Walt Whitman's "Song of Myself" is wild poem reveling in the exploration of the concept of self. Indeed, for its time, the poem was incredible radical, asserting a concept of identity in which the individual is a constantly changing entity that defies any singular definition. The above lines are perhaps one of the poem's most famous quotes, as they encompass the work's central theme of a self that transcends easy translation.
Prior to these lines, Whitman has spent the poem discussing the extent of his identity, declaring the he is "large" and contains "multitudes" (1326). Therefore, the whole course of the poem has been spent asserting an infinitely complicated self that has many definitions, but no single, definitive, meaning. Thus, the above quote, and especially Whitman's famous "yawp," is the ultimate expression of this infinite self, as he essentially asserts his ultimate independence and haughtily defies all attempts at categorization. The "barbaric" aspect of this yawp is especially important, as it highlights Whitman's wildness and his disassociation from society. In a nutshell, then, these lines are a confident declaration of a sense of self that defies society's efforts to define it.
One of the major statements the speaker makes in this long poem is that he is essentially intertwined with nature and nature (and the entire universe) is intertwined with him. In short, everything is connected.
When we think of a person or a "self," we think of an individual, a singular self differentiated from all the other selves. In "Song of Myself," the speaker celebrates him(self) but also speaks as having transcended the self. Therefore, any previous notions of an isolated self do not apply. The speaker is therefore not limited by the notion of an isolated self; he has transcended the notion. He is also therefore, not "tamed" by his "self."
Since it is uncommon to speak of one's self as having transcended the notion of self, his way of speaking is untranslatable (having never been heard or translated). This could also mean that what he is saying is unique or new and is therefore like a new language; as yet untranslatable since no one has ever heard it. Whitman/the speaker also celebrates the physical pleasures of life so this "self-transcendence" is not a transcendence over and above the physical; rather, it is a different way of looking at a self.
He compares his proclamation to a "barbaric yawp" - natural and genuine. A "yawp" to a human might sound like a dog's bark, and is therefore, untranslatable. This also means that it is open to interpretation, contradictions, and multiple meanings and impressions. Earlier in the poem (Section 51), the speaker claims:
Do I contradict myself?Very well then I contradict myself,(I am large, I contain multitudes.)