An axiom is a statement that is considered true without proof and is used as a foundation for building theories in mathematics, logic, philosophy, and other sciences. In other words, an axiom is a primary rule that we accept as obvious or so basic that it does not need to be proven. Based on axioms, the entire system of knowledge in a particular field is built.
Imagine that you are playing a board game where there is a set of rules, and everyone agrees that these rules are fair and accurate. Similarly, in science and mathematics, there are axioms that are accepted as true without proof.
To talk about what an axiom is, a simple example can be given. In mathematics, there is an axiom that through two points, one and only one straight line can be drawn. No one will dispute this because it is self-evident. Or another example: in geometry, there is an axiom that the sum of the angles in a triangle always equals 180 degrees. This is simply accepted on faith and underlies many other mathematical statements.
Thus, an axiom is something on which more complex ideas and theories can be built. It is a foundation that is trusted without the need for additional proof. In short, an axiom is, in simple words, what helps us understand the world and make predictions without spending time verifying the obvious.