MTK stands for MediaTek, and MediaTek is a well-known chip manufacturing company based in Taiwan. They specialize in manufacturing the Central Processing Unit (CPU chips) for wireless telecommunications, HDTVs, DVD etc. MTK is the largest suppliers of CPU chips to the Chinese mobile tech markets. When we say MTK phones, we are talking of phones that make use of MTK chips.
If you ever need to access the anti-theft feature on these China brand phones (e.g. Ulefone, Elephone, Zopo, Coolpad etc), here is the screenshot that may be your lifesavers: