The simplest type of interpolation is linear interpolation, where you estimate a result by comparing a data point on either side. Interpolation is a way to estimate a result based on existing data at a point where no data is available.
To perform linear interpolation at a specific point, there must be a data point available on each side. The closer the points are, the more accurate the answer will be. This method relies on the fact that the slope between the two existing points and the slope between either of the existing points and your interpolated point will be the same. By setting the slope of each of these sets of data points to be equal, simple algebra can be used to solve for the one unknown result.