Yanfly Engine Plugins is a plugin library made for RPG Maker MV, a wonderful piece of software to help you make that role playing game of your dreams. You can find out more about RPG Maker MV here.
The RPG Maker MV 1.5.1 update added a lot of wonderful new features to MV as a whole. However, the source code has been updated in a way that favors mobile game optimization over desktop games. Because of this, RPG Maker games running off the 1.5.1 source code tend to have strange issues when images are loading unlike versions 1.4.0 and below.
Yanfly made a version update to the 1.5.1 source code by meshing it together with the 1.4.0 source code, taking the best of both worlds for desktop optimization. This way, for developers who aim primarily at the development of games for desktop, your games will be running as they did with the 1.4.0 source code while being able to make full use of the new 1.5.1 features!
Happy RPG Making!