So the comb becomes positive - there's more protons than electrons. If you then put the comb near something negative, it will be attracted to it. Water is a special material. As I've already said, most things are balanced - with equal numbers of electrons and protons, so they wouldn't be attracted to the comb. But water is different.
Water is made up of two different atoms called oxygen and hydrogen.
When these two atoms join together, they share their electrons. This is often how things join together, by sharing electrons. But they don't share them equally. The oxygen atom likes electrons more than the hydrogen atom so takes more than its fair share of electrons. This means the oxygen is slightly more negative than the hydrogen.