A filter is also a curve when using smooth joints (bezier curves or splines), if you're familiar with the way curves work in Photoshop. If not, I'll try to explain:

When you use a function, a linear filter is applied to it.  Like the function, the filter goes from -1 to 1. At each point of the evaluated function, the corresponding filter point returs the exact same value, for example if  your function at 0.5 in X axis and 0.5 on Y axis returns, say 0.5 (this value is arbitrary, it doesn't mean anything without a context), with a linear filter, the value will remain 0.5, but if you add a filter point at this position and move it vertically from 0.5 to 1, the corresponding value of the function will be modified with the same magnitude, so the 0.5 value becomes 1, due to the filter.

As a filter is also a curve when using smooth joints, it can be used to smooth out an animation keyframe, and for example smooth out a jerky camera movement.

Hope this makes sense....!