The dew point is the temperature the air must cool to for its moisture to start condensing — turning into dew, fog or cloud. The closer the air temperature is to the dew point, the less headroom is left before condensation begins.
A METAR shows both numbers together: "14/08" means +14 °C air and a +8 °C dew point — a spread of 6 °C is fine, while 1–2 °C is not, making fog, low cloud or icing likely. A spread narrowing toward evening means fog risk.