How do I interpret humidity and dew point?

Humidity is a percentage: 100% means the air is completely saturated with moisture, while lower percentages mean drier air. Dew point is the temperature at which air would become saturated if cooled — it's independent of actual temperature. Dew point is often more useful: a dew point above 70°F feels oppressive regardless of the actual temperature, while a dew point below 40°F feels dry. High humidity + high temperature is uncomfortable (sweat won't evaporate). High humidity + low temperature can mean fog or drizzle.