Hardwood is a deciduous, broad-leafed tree or wood that comes from a deciduous, broad-leafed tree — in other words, a tree that loses its leaves each year. Examples include maple, ash, mahogany, and other woods/trees commonly used for constructing musical instruments such as guitar and bass.
There is no correlation between a wood being technically classified as a hardwood and the wood actually being “hard” or dense. For example, basswood is classified as a hardwood, but is in fact quite a soft and low-density lightweight wood.