Tocada is the past participle of the verb tocar, which literally means "to touch." However, in Brazilian Portuguese, especially in informal contexts, "se tocar" means to realize something, to become aware, or to get a clue.
In the lyrics, the speaker says, "Eu queria que você tivesse se tocado à essa altura" (I wish you had realized by now). This phrase implies a frustration that the other person hasn't understood the situation or the speaker's feelings, making tocada a key word for understanding the emotional core of the song.