This video demonstrates how to create a rainbow shader effect using Three.js's TSL (Three.js Shading Language) instead of GLSL. The speaker refactors a pre-existing GLSL shader, migrates it to use the WebGPU renderer, and then implements the same effect using TSL, comparing the code lengths and functionality of both approaches.
Based on the transcript, positionLocal is described as the local variable for position within the TSL shader. It's a vec3 (a three-component vector) representing the X, Y, and Z positions along the surface of the rendered object, encoding these positions as RGB values to determine color. The speaker explains that using positionLocal in the fragment node essentially assigns a color based on the object's surface position.