Shading units (or stream processors) are small processors within the graphics card that are responsible for processing different aspects of the image.
Graphics cards have dedicated units for different types of operations in the rendering pipeline such as pixel shading, vertex shading, geometry shading. This means that the more shading units that a graphics card has, the faster it will be able to allocate power to process the workload.
Most graphics cards today have between 749–1,496 shading units. The Sapphire Pulse Radeon RX 5500 XT 8GB is a great example of this, with 1,408 shading units. Top-performing GPUs will have over seven times the amount of shading units. The Asus Dual Radeon RX 460 comes with an impressive 8,963 shading units.