Nato
In short
An intergovernmental military alliance of 32 member states from Europe and North America. It operates on the principle of collective defence to ensure the security of its members.
In plain words
NATO is a group of countries that promise to protect each other. If one member is attacked, the others will help defend them. This keeps the group and its members safe from outside threats.
A simple example
If a NATO member is invaded by another country, all other members are treaty-bound to assist in defending that nation.
Why it matters
It provides a massive deterrent against military aggression in the North Atlantic region. The alliance ensures that small nations have the backing of powerful militaries if they are threatened.
Easy to mix up
Do not confuse NATO with the European Union, which is primarily a political and economic union rather than a military alliance.