What is a coelom ? and why are some invertebrates described as acoelomates or pseudocoelomates​

The main difference between acoelomate and coelomate is that acoelomate is an invertebrate that doesn't have a coelom whereas coelomate is an invertebrate that has a true coelom.

A coelom is a fluid-filled body cavity, which is completely lined by the tissues derived from the mesoderm.

The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs.

Unlike coelomates (eucoelomates), animals with a true body cavity, acoelomates lack a fluid-filled cavity between the body wall and digestive tract. Acoelomates have a triploblastic body plan, meaning that their tissues and organs develop from three primary embryonic cell (germ cell) layers.

Pseudocoelomate animals have a pseudocoelom (literally "false cavity"), which is a fluid filled body cavity. Tissue derived from mesoderm partly lines the fluid filled body cavity of these animals. Thus, although organs are held in place loosely, they are not as well organized as in a coelomate.