What does it mean when a system or a reaction is endothermic?

It means the system absorbs energy from surrounding objects; usually, but not always, involving heat

When a chemical reaction is endothermic, it means that the energy needed to break the bonds in the reactants is greater than the energy released when all of the new bonds are formed in the product. During an endothermic reaction, the reaction will absorb energy from its surroundings.

This is the opposite of an exothermic reaction, where the reaction releases energy because the energy needed to break the bonds in the reactants is less than the energy created in the product.