Atoms consist of protons, electrons and neutrons
They join each other to form structures of different substances according to the existing physical conditions, such as: temperature. If a metallic body in a vacuum is not subject to the influence of an electric field, its positive and negative charges are evenly distributed, and it is concluded that the body is neutral. Look at picture # 1
If you then consider that there is an electric field E near the metal, the metal negative charges (electrons) move under the influence of the field E. As a result the charge distribution within the metallic body changes, part of the metal is positively charged and the other part becomes negatively charged.
However, taking the metallic body as a whole, it remains neutral, as the number of charges has not changed. Look at picture # 2. The charges try to distribute itself on the surface of the metal, as a result, we have an electrically charged body that attracts and repels other bodies. Under these conditions, it is said that the metal is polarized.
If the electric field is removed, the metallic body returns to its previous state. What this means is that the electric field has the ability to alter the distribution of electrical charges inside a body. Look at picture # 1
Take note that this concept applies to a conductive body. The non-conducting bodies, called insulators or dielectric, can also be polarized, but this polarization occurs at the level of molecules or groups of molecules.