Words. They cannot be used to explain reality beyond the brain; nevertheless, they are also the only way to describe it. This may be a silly paradox, however it tries to make evident the following: we are trapped inside our brains and nothing else exist. Words exist only inside our brains, no matter how obvious we find them to be related with “real things”.
The objective, however, is not to try to understand what is outside our brains, since this is impossible. Suppose someone actually finds it, then it immediately becomes an idea, a sequence of words, therefore, something inside our brain. The objective is rather to better understand how we build these ideas, how they pass from brain to brain and how they affect each one of us. Then we limit the reality to all that can be thought, and understanding how we think means also to understand the reality.
Brains model their inputs. The model is based on internal connections created such that the inputs can be characterized and analyzed in a useful way. Useful in the sense of life, this is, to promote the creation of more brains. Patterns on the input (no matter their origin) can be represented by a sequence of symbols, words. The strength of these patterns, and the fact they can be shared with other persons is what suggests they originate outside, in the “real world”. Symbols sequences representing strong patterns are called, by us, "true".
Now, just notice: the previous paragraphs along with this sentence is just a sequence of symbols.
