In The Holy Bible, the Merarites were descendants of Levi, who in turn was one of the 12 sons of Jacob.
In Numbers, in the 25th Chapter, Levi's descendants are given as Gershon, Kohath, and Merari. From these sons came the Gershonites, the Kahathites, and the Merarites.
The tribe of Levi was the priestly tribe of Israel, designated by the Lord to serve Aaron and his sons, and were also charged with the religious practices and artifacts of the nation.
Rather than being the group that was in charge of toting the Ark of the Covenant around the desert, the Merarites were given the more mundane task of schlepping around the other hardware of the mobile temple such as the framework, support poles, brackets, and other parts of the tabernacle.
Sources:
http://bible.cc/numbers/26-57.htm
http://www.biblebrowser.com/numbers/3-36.htm
BrevityQuest 2009