A simpler way to understand what others are saying here would be "cause and effect." It's possible that everything you think and do is a reaction (effect) to something else that happens (cause) in a completely predictable and unchangeable way.
Your thoughts are electrical currents and chemical reactions in your mind. Your actions are electrical signals sent to your body from your brain, thus thoughts are the only things that actually matter in the context of free will. It is possible that these electric and chemical signals are all only released as effects of direct causes rather than by your choice. It is possible that everything you ever have done and ever will do is a series of reactions. You cannot know for sure because you don't know the intimate inner workings of your brain, you only know what you perceive (which is very untrustworthy as can be observed by asking a dozen witnesses of an event exactly what happened and receiving rather different answers).
Because this is a possibility, that everything you do is reactionary, it's impossible to claim free will as an undeniable fact.