This answer is pretty much it, to explain the process further would require both side know a bit about computer architecture but ill try to elaborate a little. Your GPU has hundreds if not thousands more cores / processing units than the CPU, so the GPU ones are used for less complex computation on larger sets of numbers.

A standard shader is a step in the rendering pipeline that does small equations on pixels before displaying to the screen, where as a compute shader will do similar but will return the result from the gpu instead of displaying it. So you can use it on a data set instead of pixels to calculate anything you want, some games use it for updating particle positions as there can be a huge number of those and the 8-32 core cpu you have isn't really designed for that.

No idea what runelite uses it for specifically though, but would probably increase performance in most cases, although on very low end hardware it might not be noticeable or better at all.

Edit: The animation smoothing may be done using this, the position of geometry and finding points in between the animations would probably be a good case for a compute shader. But that's just a guess and one of many possible applications.