Interpolation is an estimation of value between two known points. Intuitively, interpolation (green dot on the image) is somewhat safe due to the range provided by the known surrounding points.
Extrapolation is an estimate of a value outside the range of known data points. Domain knowledge about the phenomena can make it safe to use - you know the relationship holds for some range outside the data points. Without that knowledge, you get deeper into trouble the further you go from known data points.
Extrapolating values for the orange dot on the right is potentially more dangerous than the left one due to the proximity to the other (blue) data points.