Figurative language uses metaphor and simile to convey ideas and feelings and to give descriptions. Figures of speech are not to be taken literally but can communicate pictures and emotions clearly and poetically, and song lyrics are an ideal place to find them.
The very title of this song is figurative language in the form of a metaphor. The songwriter is not saying that he is an actual, literal rock. After all, rocks cannot sing or write songs! He is saying that there is some important way in which he is rock-like. "I am a rock" could mean "I am gray and cold" and/or "I am solid" and/or "I will not be moved."
Within the lyrics of the song, Paul Simon pairs his "I am a rock" metaphor with the line "I am an island." These two metaphors help illuminate each other's meaning. Simon is using figurative language to describe feeling isolated and hardened. We also can clearly understand his meaning from other lyrics in the song where he says more literally, "I have no need of friendship, friendship causes pain."
We see additional use of figurative language in the song when Simon speaks of a "fortress," "armor," and a "womb." All of these nouns are used in metaphors, which can be translated into literal emotional terms in line with the primary metaphorical theme of the song - "I am a rock."