What do electrons do in metals?
Electrons do stuff in metals. Principally, they are responsible for the malleability and ductility of metals, and for the ability of metals to conduct electricity and heat.
The often quoted description of metals is as " positive ions in a sea of electrons ". What does this mean?
Typically, metals are described as an infinite array of metal atoms, close-packed together. Each metal ATOM contributes 1 or 2 (or more) electrons to this lattice of close-packed positive ions (these are positive because they have lost 1-2 electrons). The valence (the outermost) electrons are free to move and are delocalized across the lattice, and are the "glue" that holds the lattice of positive metal ions together.
Because this electrostatic bonding, this metallic bonding, is non-localized, and not confined to a particular metal atom, the metal nuclei may move with respect to each other, while maintaining a strong interatomic force (the metallic bond). Also because the electrons are delocalized, electric charge can be propagated through a metal; i.e. metals are conductive to electricity. Heat can also be propagated by electron movement; metals are also conductive to heat).
Because the positive metal nuclei can move with respect to each other, the metal is MALLEABLE. It can be hammered out into a sheet. It can also be drawn out into a wire; it is DUCTILE. Malleability and ductility are prized properties of metals, in that we can use these properties to make sheets and wires, which can be further be used for machinery and tools and automobiles and other stuff. They say that a lump of gold can be hammered out into a sheet only a few atoms thick (gold is so malleable).