I am not able to understand the definition of metallic solids in our book: "They are orderly collection of positive ions surrounded by and held together by a sea of free electrons. The electrons are mobile and are evenly spread out throughout the crystal." Please explain.

 
Metallic bond can be described by electron sea model. According to electron sea model, metal ions are packed densely and the electrons of each metal can be shared by all other nearer atoms. Electrons are not fixed to any particular atom. Thus electron makes a sea of electrons. These electrons occupy the space between metals, but can be moved by application of external energy like heat, electricity, etc.