The above song, a one-hit wonder from 1971, is a particularly historically and culturally situated lyrical consideration of signs — in the denotative sense, surely (actual posts and placards containing written messages), but also in various connotative senses.
Semiotics, as outlined in the Danesi reading, can be a lot to get your head around. If you'd like more in-depth background on signs, codes, texts, and more, look to this excellent primer, "Semiotics for Beginners," particularly parts 1, 2, 7, and 9.
Participation! Consider this image below, indicating the appropriate gender of restroom facilities. Does it look more like a man than a woman? Why? What is indexed? What is symbolized?