They are numbers counted or measured in the opposite direction. Usually a reference number can be matched to a reference point on a number line. A simple physical interpretation works for temperature. One of the temperature scales, Celsius, uses the freezing/melting point of water as zero degrees. We can often find or measure a temperature of some positive amount, and if we do, this is usually of a temperature at which water is not a solid (not frozen). We naturally assume these are positive values, to the right of zero on the number line. A NEGATIVE Celsius temperature value is one at which water is typically in the form of ice, and the number is to the left of zero on the number line, and the number is given a negative sign next to it.
Example: -5 degrees Celsius is colder than the freezing point o f water and this water would be ice. 4 degrees Celsius is cold, but water at this temperature will be liquid.