For example, we can look at the variations of temperature by the time of the day.
We can write it in a two column table, where we can write the hour in one column and the temperature in the other column.
This will show us a relationship between them that is oscillating.
We can graph this and have something like:
Then, we can adjust a function to that, like a trigonometrical function that can model this relation between temperature and hour of the day. There you wil have an expression for this pattern.