Assume an alien has landed on Earth and wants to learn English with the help of an English Dictionary.
He looks up the meaning of "the".
Meaning of "the":
"denoting one or more people or things already mentioned or assumed to be common knowledge"
Now he searches the individual meaning of each and every word of the above sentence.
If he continues to do this infinitely, he will land on many words. The words uncovered may be referred to by visiting the dictionary manually 1 by 1.
Obviously, in this exercise, there would be nothing but circular references of word meanings. To resolve this issue, we may declare few words as "axioms".
What would those sets of words which would be axioms be?
Using these set of words only(or predominantly), one could understand every other word in the English dictionary.