If there was no resistor(pull-up or pull-down) to +5V or to ground, the input pin would be "floating." That means electrical background noise could make it be read as a "1" or a "0"(read posts above me). The resistor to +5V or ground keeps that pin at that voltage(+5V or 0V, "1" or "0") through a resistor until the button is pressed. This keeps it from changing states erratically. When the button is pressed, it makes a short to +5V or ground, and the current/voltage from the other portion(the pull-up or pull-down) of the circuit is negligible.