Water also known by its chemical formula H2O. It is composed of three atoms - Oxygen (O) and two hydrogen atoms (H). The polarity of each end of a single water molecule plays a part in how well water is accepted (reacts) in a chemical reaction. One side (O) has a slightly negative valence or charge while the other side (2H) has a more positive valence or charge. This is due to electromagnetic resonance (distribution of electrons). When chemically reacting, the water molecule will tend to split up releasing one hydrogen atom (H+) in solution. The remaining atoms form what we call a hydroxyl (OH- ) which is highly reactive (electron needy).
With all this in mind, the "universal" attribute of water results from its ability to provide both positive (H+) and negative (OH-) ions in solution making it most commonly used to trigger reactions with other chemicals.
High (H+ ) concentration makes the solution more acidic whereas High (OH- ) will make the solution basic in characteristic. Research acids and bases for more info.