Biomass fuel refers to any source of organic material. Organic material is created through the life that exists on earth be it animal or plant. For the majority of Biomass Fuel power plants the main fuel source is plant material.
The sun is a massive nuclear reactor giving off vast amounts of light and heat energy. Life on Earth uses this source of energy to thrive by transforming the suns energy in to cell growth and reproduction. Trees for example grow and produce leaves that harvest and use the suns energy to photosynthesis and thus produce energy for growing and creating new cells as part of the tree structure growing new branches for further harvesting and increasing their trunk size to accommodate more branches and more energy sources.
This growth can be converted into a biomass fuel that can be burnt by us to provide a heat source that can then be converted into electrical power through use of steam turbines spinning to produce electricity.
This is a technology that has been used extensively in the 20th century with coal and oil burning furnaces providing the heat source for the steam production and source of power to spin the electricity creating generators.