 So let's look at this expression. Suppose an object is stationary in your reference frame. So it's not moving, and in that case its momentum is zero. Our expression says that the object will still have some energy, and this is called the rest energy of the object. Now this energy is given by E equals mc squared, which is Einstein's famous equation. So what this tells us is that all matter, everything that we can see, can be thought of as frozen energy. And the conversion factor of mass to energy is huge, it's c squared, so a tiny little bit of mass corresponds to a huge amount of energy. It is this principle that makes nuclear energy so powerful, you just need a little bit of mass to generate massive amounts of power. And for the same reason, this is why atomic bombs are so frightening. The bomb dropped over Hiroshima in 1945, converted an amount of mass equal to about a fingernail into energy, but just that fingernail's worth of mass was enough to vaporize an entire city. So what about on the other hand, for something like a photon, a particle of light which has no mass, in this case we get the expression e is equal to pc, or in other words, p momentum is equal to e divided by c. This tells us that something like a photon which has no mass can still have momentum. This seems really strange, but it's a prediction that also comes up in quantum mechanics. c is a very big number, and so a single photon has only a tiny bit of momentum, but the cumulative radiation pressure from all the photons being emitted by the sun can have a considerable effect, and spacecrafts traveling to Mars, for example, will typically be displaced by thousands of kilometers, and the effects of this have to be taken into account when planning that trajectory.