Energy is the capacity to do work. Scientifically, there are two main categories of energy: potential energy and kinetic energy. Potential energy is the energy you get from the mass of the object and height. The higher you go, the more potential energy you have. Likewise, the heavier an object, the more energy it carries. For example, if you drop a rock on your foot from 6 inches, it wont hurt that bad. But if you drop it from 6 feet, you will probably break your foot. This is because the 6 foot rock had more energy to break your foot. What about when we move? This is what kinetic energy is: the energy from moving. If a car hits you going 1 mile per hour, you can probably walk away just fine. However, if that same car hits you going 100 miles per hour, you will probably die. Likewise, heavier moving objects also carry more energy. If I throw a pebble at a window, it probably wont break. If I throw a heavy rock at it at the same speed, it will probably break.
For something to get to space, it has to sit on top of a rocket. This rocket will of course end up traveling very fast at thousands of miles per hour very high in space. Of course, this means that we would have to use a lot of energy to get anything to space. Depending on how you calculate it, getting 1kg into low earth orbit(LEO) takes about 100MJ of energy. To compare, this is about $3 of electricity. But wait, if it only takes $3 worth of energy to put 1kg in LEO, why does space travel still cost millions of dollars? This is because the have to launch the weight of the rocket itself, which is extremely heavy. Typically, what makes it to orbit is less than 10% of the weight of the rocket!
oh no rocket science! 