Let’s examine what happens when acetic acid is dropped into the water. The molecule of acetic acid is able to transfer one of its protons to the water molecule. In this way the equilibrium will be shifted in favour of oxonium ions. Acidity of the solution is caused by the oxonium ion. The more acetic acid can be found in the water, the more oxonium ion is produced, and hence the more the solution will be acid.

Well, acid is the compound whose molecule can pass proton over the water molecule.