A function is a special kind of relationship.

In this relationship, each input value (called the domain) matches with just one output value (called the range).

Here are some important things to know about functions:

1. What is a Function?
   A relation named $f$ on a group of things called $X$ is a function if each element $x$ in $X$ connects to only one element $f(x)$.

2. Domain:
   This is the collection of all possible input values.

3. Range:
   This is the collection of all possible output values.

4. Vertical Line Test:
   If you draw a straight vertical line on a graph and it touches the graph at more than one point, then it is not a function.

In simpler terms, a function keeps things organized by making sure each input only has one output!