what is triploblastic and what is advantage of being triploblastic

Animals in which the embryo have three embryonic tissue layers or germ layers i.e. ectoderm, mesoderm and endoderm are termed as triploblastic. 

These germ layers are formed during development of the bastula into gastrula. Organisms which develop from such an embryo are known as triploblastic organisms. All the three different layers have different functions and develop to form different sets of organs and structures. The nervous system and skin are developed from ectoderm germ layer; the digestive structure is
developed from the ectoderm; muscles and other parts of the body from the mesoderm which is a layer present between the ectoderm and endoderm.

Before triploblastic organisms developed, the organisms had only two germ layers (ectoderm and endoderm) and were known as diploblastic. The advantage of triploblasty was that it helped in evolution and development of advanced organisms with better morphological and physiological features. Various things like skeleton and circulatory system developed from the third layer called mesoderm.