caps your fps at your monitors refresh rate, typically 60 fps
since your monitor can only refresh so many times per second, any frames over this amount are technically wasted, and can put unneeded stress/wear on your gpu
It stops the monitor from rendering a frame until it's fully drawn. This prevents screen tearing, but can bring down your FPS to certain values (numbers like 30 or 60.) Only use it if you're experiencing tearing, else you're just hurting your performance.