Determinism is the belief that all events are a result of physical interaction between particles, energy, and forces rather than Chaotic (as in entirely random events/values that don't follow any pattern and logic as have no cause) somethings sending the Universe into a storm of craziness.
Here's how I explained it to my friend:
Well if you think about the way things happen... if you put two particles in a vacuum a certain distance apart, you may predict what is going to happen, how they are going to interact (with a margin of error caused by lack of understanding). If we take into account that EVERYTHING, including people's brains, are made out of fundamental particles... if you knew every involved value and factor at any point in time, let's say, the Big Bang, then you could predict exactly what is going to happen in the future (you could predict infinity years ahead... until the end of time so to speak).
That means you could predict exactly what choices people are going to make. The brain works like a machine, taking in information and spitting out responses, like a computer trying to solve a problem. When someone makes a choice, it is impossible that they could have made any other choice.
We must understand that since our brains work like this, you can only imagine it as if you were an Observor standing outside of your body and viewing everything as tiny particles... ACTION = REACTION