The latest Steam client update introduces a feature aiming to stop the longer load times and framerate stuttering that can happen the first few times you load a game after installing it. It'll do this by trying to download and pre-cache the game's shaders, part of their technoguts which your own computer can need to compile when you first launch it. That compilation is what can cause the hold-up. This doesn't affect every game and isn't a huge problem but hey, anything that makes getting into a game smoother is welcome. You shouldn't need to do anything to benefit from this.
Let's get mildly technical. This is about shaders, little bits of code in games which mostly power visual effects from special effects to lighting and, well, shading. Some games need to tailor these to your computer, compiling them when you first run it, sometimes at all once or sometimes when needed. This can mean hitches as you play or a bit of a wait. Those are the games this new Steam feature targets.
Valve explain Steam's new shader pre-caching in the update notes:
"Whenever possible, depending on hardware and driver support, Steam can download pre-compiled shaders for your specific video card. This reduces load times and in-game stuttering during the first few launches of OpenGL- and Vulkan-based games on supported hardware. This feature may use a small amount of additional bandwidth as Steam uploads and analyzes a shader usage report after each run of the game."
If you don't want that, or if it causes problems, you can disable the feature.
It's not mega-huge but hey, if this works for many games I'll be happy to have those little hitches and waits removed.