The older Arrhenius definition said a base must release hydroxide ions in liquid water. The Bronsted-Lowry definition focuses entirely on the movement of a single proton. A proton is simply a hydrogen atom missing its one tiny electron.
This broader rule works for chemicals dissolved in water or other strange liquids. It even works for chemical reactions happening in a totally dry gas phase. When a base accepts a new proton, it changes into a new chemical.
Chemists call this newly formed chemical the conjugate acid of that base. Together, the original base and the new acid make a conjugate acid-base pair. A molecule must have an available pair of unshared electrons to act as a base.
These unshared electrons reach out and grab the incoming positive hydrogen ion. This action forms a strong new chemical bond between the base and the proton. A common student misconception is thinking a base must have a negative charge.
Many neutral molecules like water and ammonia function perfectly well as Bronsted-Lowry bases. They just need that available pair of electrons to catch a passing proton. This makes the Bronsted-Lowry concept incredibly useful for everyday chemical predictions. It helps scientists understand how different medicines and foods behave inside the human body.