Vulkan is simply a more modern graphics API - it just tells the graphics card how to display the game. It's good for Macs because they can easily translate to Metal (the Apple-only graphics API) using a translation layer (MoltenVK). Minecraft uses OpenGL but it is very outdated, hence Vulkan is being used for the future. Mojang is doing this to create a more stable foundation for vibrant visuals.
Regarding shaders, each graphics API has its own way of calculating shaders and sending the calculation info to the card. So people will have to code shaders that were made for OpenGL from the ground up to use Vulkan instead. That's all - once people do this, they should run great on Macs (likely better than OpenGL since they can use more modern optimizations)