The term 'bagel filling' refers to the various ingredients that can be used to fill a bagel. Common fillings include lox (smoked salmon), EGG, and cream cheese. Lox is a type of cured salmon, usually served in thin slices. EGG is a versatile ingredient that can be boiled, scrambled, or poached. Cream cheese is a soft, spreadable cheese made from cow's milk. All three of these ingredients are popular choices for bagel fillings.