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The definition of linear is soemthing that is represented as a straight line or has a constant rate of change 

Let's say you were at a gas station and 1 gallon of fuel is 5 dollars well and you made a graph about it and marked the gallon's bought x and the dollars spent y everytime you filled your tank up with a gallon of fuel you would lose five dollars. It would look something like this. 

1 gallon loss of 5 dollars 

2 gallons loss of 10 dollars  

3 gallons loss of 15 dollars 

4 gallons loss of 20 dollars.

Non-linear means to ahve not a costnet rate of change as in let's take driving to another state every hour you spend will not equal the same aount of miles as the previous because you may have to stop at traffc lights use the restroom and eat so it woul look something liek this 

1 hour 55 miles

2 hour 70 

3 hour 30 

4 hour 66