A scatter shows whether two measures move together. A linear trendline is a regression Excel fits for you, its slope is the direction, its R² is how tightly the points hug the line. Pick a dataset, predict the fit, then reveal it.
Cycle through Anscombe I–IV: every one has the same mean, variance, correlation, and trendline, but four completely different shapes. The trendline is a model of the data, not the data. Plot before you trust a number. Then compare the Strong / Weak / None sets to feel what R² is measuring.