What is this tiny plastic insert in my sliding window sash?
By looking at the space between the window frame and the sash (approximately 1/4 inch ?) I believe those shims are installed to keep the sash straight and from rattling around when it is opened and closed.
Or they could be part of the spring assisted lift system of the window. The piece that is in view may be attached to a mechanism on the edge of the window which compromises the spring lift mechanism.
I'd say some sort of shim, though.
It may be easier to find the manufacturer of your window and get the shims from their site.