What is a Trendline in tableau?

A trend line is a fitted model drawn over the marks in a view, showing the underlying direction of the relationship rather than the point-to-point movement.

Add one from the Analytics pane. The available models are linear, logarithmic, exponential, power and polynomial, and the choice should follow the shape of the relationship rather than whichever looks tidiest.

Hovering over the line gives the equation, the R-squared value and the p-value. R-squared indicates how much of the variance the model explains; the p-value indicates whether the relationship is statistically significant.

Two things worth saying about them. A trend line describes correlation, not causation. And a high-order polynomial can be made to fit almost any scatter of points — a good fit on the sample is not evidence the relationship will hold.