A water molecule is charge neutral because there is the same number of positive charges as there are negative charges.
In this diagram, called a Lewis structure, the dots represent electrons while the lines or dashes represent a covalent bond of two electrons.
When water ionizes one of the hydrogen atoms absconds with itself and leavesit's electron behind, giving us the hydroxide ion. The extra electron gives hydroxide a net charge of -1.
The brackets indicate that this is an ion, charge is denoted at top right.
To go deeper down the rabbit hole on this one I recommend reading up on the Octet rule and Electronegativity.