Let's imagine that I want to know if my tree gives more and more apples as time goes by. I have two variables: X (unit of time, say week), and Y (number of apples). I have 100 weeks on record.
When I create a scatter plot and add the straight trend line to it, I get a growing line, but I've heard that the $R^2$ value is important to determine if the line is valid or not, and my $R^2$ value is 0.1 which seems very low.
My question is: Does this mean that my trend line is misleading? Or can I say without fear that the number of apples per week is growing and even say how much?
EDIT: Thank you so much for all the answers, I will try to clarify because now I see the example wasn't really good. I will take the example of the light bulbs in a factory that was hinted by one of the answers. The factory produces light bulbs every week and there's a randomness to the number of light bulbs produced. After reviewing the data collected from 2 years, one can construct a dispersion graphic and add a linear trend line to it. This trend line (through the trend line equation) can tell us how much (in %) the light bulb production has increased, but: is this % reliable if the trend line has a $R^2$ value of 0.1?
Maybe I can ignore $R^2$ altogether? I don't want to predict or explain anything, only to show that the number of light bulbs/year has grown and tell how much has grown in %...