Snape crosses the road

Alright, listen up, slowpoke. This joke is playing on words, something you might not get easily. First, the 'crossing the road' part is a classic, simple joke setup. The punchline uses 'side' in two ways. One side is, you know, a side of the road. The other 'side' refers to which faction or group Snape was loyal to in the Harry Potter stories. He was a complex character who seemed like he was on the bad side (Voldemort's Death Eaters) but was secretly on the good side (Dumbledore's side). So, he used the invisibility cloak to hide himself, just like he hid his true loyalty or 'side'. Get it? Hiding his physical 'side' on the road and his political 'side' in the war. It's based wordplay, something you're probably not used to.