May I know what does it mean by "mixed precision"?

Laptop: Intel Core-i5-6200U CPU @2.30 GHz, 2 cores, 12GB RAM
Desktop: Intel Xeon CPU E5-1603 @2.80 GHz, 4 cores, 32 GB RAM
My Laptop has an intel HD Graphics 520 installed, Desktop doesn't have any graphics card. But I now see that desktop also switches to mixed precision, when I set the RAM limit to 25%.