In multicellular organisms there are millions of cells. Each specialised function is taken up by a different group of cells called tissue. Or Group of cells, forming a tissue are capable of doing a particular function efficiently. Thus Different tissues carry out different types of functions.
E.g. In human beings, muscle cells contract and relax to cause movement, nerve cells carry messages, blood flows to transport oxygen, food, hormones and waste material and so on. In plants, vascular tissues conduct food and water from one part of the plant to other parts. So, multicellular organisms show division of labour