Spikes are an obstacle in Braid. If Tim touches a spike, then he "dies" (see Time Manipulation). Spikes usually are depicted graphically with fire.
This is a reference to Mario. Instant death was portrayed in a few different ways. One was spikes, the other was fire, the last was simply a hole that you didn't want to fall into. However Braid is completely outside of that mind frame, since death is completely reversible, and sometimes even required.