A linear function has the form y=mx+b where x is an independent variable and y is a dependent variable.
So you have to think of a linear process that relates two variables.
Here's one: conversion of temperature in Fahrenheit degrees to Celcius degrees. C=(5/9)(F-32). With this linear formula you can convert any temperature from F to C. So let's take 77ºF. What's that in degrees Celcius?
The formula says take off 32: 77-32=45, then divide by 9: 45÷9=5, then multiply by 5: 5*5=25, that's 25ºC.
You can use a linear formula to convert US dollars to UK pounds or UK pounds to US dollars.
You can convert inches to centimetres, miles to kilometres, gallons to litres, and so on.
You can work out cellphone (mobile phone) charges according to a plan that has a fixed charge and a variable charge depending on the number of calls you make, or their duration.
Taxicab fares are similarly calculated. There's a fixed charge and then a cost per mile.
Get the idea? Perhaps you can think of a few yourself now.
A linear function can have more than one variable: how much is 2lb of bananas and 3lb of oranges, when you know the cost per lb of each fruit?