When an atom loses an electron, its overall charge becomes more positive by one. For example, a neutral atom would have a charge of positive one after losing an electron.
All atoms tend to seek to be stable, which occurs when they have a neutral charge. When atoms don't have a complete outer orbital of electrons they will have a negative or positive charge. In order to become neutral, they will seek to either gain new electrons or lose electrons through bonding. In general, metal atoms often have excess electrons and will try to donate them through ionic bonding. Non-metal atoms typically need more electrons and will either accept them from metals or share electrons with non-metals through covalent bonding.