MTK means MediaTek, and it is a fabless company from Taiwan that designs and distributes technology products. What's so particular about this company? Just that the chips they manufacture are really cheap, and that they're incorporated inside most cheap Android devices. MediaTek is also the company that manufactures chips for almost all dual-SIM devices.
MTK Engineering Mode is an app that lets you activate a series of extra options on your MTK device. In other words, it lets you do more with devices from Tecno, Infinix, Elephone, Oppo, Chuwi, etc.