Dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense to form liquid water (dew). When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface. When the temperature is below the freezing point of water, the dew point is called the frost point, as frost is formed rather than dew.
The dew point is the temperature at which the water vapor in a sample of air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates. At temperatures below the dew point, the rate of condensation will be greater than that of evaporation, forming more liquid water. The condensed water is called dew when it forms on a solid surface, or frost if it freezes. The condensed water is called either fog or a cloud, depending on its altitude, when it forms in the air.
If the temp and dewpoint are the same it means the air has reached 100% saturation, the air may be clear, but caution is required because you may suddenly find yourself coping with anything from mist to thick, thick fog should the temp drop ever so slightly. Hence the rule mentioned by LRP re visible moisture and temp, the particular temp will be spelled out in your aircrafts manual.