tocar = play a musical instrument tocar em = touch something with your hand, finger etc
Let me explain with an example. In the reverse tree, there used to be this sentence: "Don't touch the plate." Translation: "Não toque o prato." There was a small uproar in the discussions because "tocar o prato" conjured up the image to native speakers of Portuguese of someone trying to "play" a plate as if it were a musical instrument.
That's the best explanation I was able to figure out.