This is the subatomic particle found in the nucleus of an atom that carries a positive charge. In the case of a hydrogen ion, it's essentially just a proton since it has lost its only electron.
Hydronium Ion (H3O+): This is what happens when a water molecule accepts an extra hydrogen ion. It's like if our solo dancer finds two other dancers and forms a trio!
An acid is any substance that donates or releases hydrogen ions into solution. Imagine an acid as the bus dropping off all those solo dancers (hydrogen ions) at the party.