Hydrogen peroxide is a very strong oxidizing agent, and is not very stable. It spontaneously decomposes into water and oxygen, releasing heat in the process. At very high concentrations (>70%) , the heat released during decomposition can actually vaporize the remaining liquid. Because it spontaneously decomposes, concentrated hydrogen peroxide has to be stored in special vented containers.
As the concentration increases, it gets progressively more dangerous. High concentrations of hydrogen peroxide spilled on clothing or other combustible materials can cause the material to spontaneously combust. High concentrations spilled on skin causes severe chemical burns.
If you drank pure hydrogen peroxide, it would kill you, and it wouldn’t be pleasant.