Different interpolation types allow inbetweens to be blended in and out in a smooth way. Take the example of two weightPositions of a weight, one at 0.5 (wp1) and one at 1 (wp2). With linear interpolation, the WPs are weighted according to the following plot:

Now consider a (trivial) plain dataPoint that simply moves a vertical bar down, like this:

No Inbetween

Adding an inbetween to get a more arced movement results in the following motion in case linear interpolation is used:

Linear

Here are two examples where the WP at 0.5 (wp1) has been set to a non-linear interpolation:

Spline Arc

There are other interpolations, too. They are described in the Glossary under WPos Interpolation.