The core idea
Air's appetite for water vapour depends entirely on temperature: warm air can hold a great deal, cold air very little. Relative humidity (RH) expresses how full that appetite is — the ratio of the water vapour actually present to the maximum the air could hold at that temperature, written as a percentage. Absolute humidity is the raw quantity: grams of water vapour per cubic metre of air (g/m³). Two air samples at the same RH can carry very different amounts of water.
The dew point follows directly. Cool a moist air sample and its RH climbs until, at the dew-point temperature, it hits 100 % — the sample holds its maximum, and any further cooling forces vapour to condense into liquid. A useful approximation: below about 30 °C, the maximum vapour capacity roughly doubles with every 10 °C of warming.
The most instructive trap in the whole subject: the amount of water in the air doesn't change when air is heated or cooled — only the RH number does. Take 1 m³ holding about 15 g of water. At 30 °C that is ~50 % RH; cool the same air to 15 °C and it reads ~90 % RH; below ~10 °C, condensation begins. The 15 g never moved — the capacity shrank.
Real-world example
A summer bedroom cooled by a wall-mounted split unit: outdoor air at 28 °C, 60 % RH (dew point ≈ 19.5 °C, roughly 15 g/m³ of moisture). The evaporator coil runs at about 7 °C. As room air drifts across it, everything touching that coil is far below 19.5 °C, so water condenses out of the air and drains away. The same air, reheated to supply temperature, now measures about 60 % RH at a lower temperature — around 12 g/m³. The unit didn't only cool the room; it dehumidified it. This condensation is precisely where the refrigeration cycle's latent heat (≈ 2,450 kJ per kg of water) is absorbed — condensing water releases roughly 586 times more energy than heating that water by just 1 °C. It is also why an oversized AC that short-cycles leaves a room cold but clammy: it shuts off before enough moist air has completed enough passes over the coil.
Common pitfall
Reading relative humidity as an amount of water. People hear "50 %" and picture the air as half-filled with moisture; then they're surprised that drying outdoor air to 50 % and heating it produces an indoor RH near 25–30 %. Absolute humidity is unchanged by heating; RH falls simply because capacity grew. Specifying dehumidification or mould prevention in % RH without stating temperature is a guess, not a specification — 60 % RH at 20 °C (≈ 14 g/m³) holds far more water than 60 % RH at 10 °C (≈ 4.6 g/m³).