If you have a set of points ( or numbers starting off, like (1,1) and (2,2) or what so ever. you have a slope. As long as you numbers continue to grow, remain constant, or decrease, you have Linear Data
Linear is a smart word for "Straight.
Think of it like this. Your parents give a dollar a day.
Day one you have one dollar
day two you have two dollars.
So if you can figure the slope of the line. you can figure how much you will have in a month!.